use overloading in ImageFrameCollection + more tests for API-s affected by the PR

Author: antonfirsov

src/ImageSharp/ImageFrameCollection.cs

@@ -198,15 +198,31 @@ public Image<TPixel> CloneFrame(int index)
             return new Image<TPixel>(this.parent.GetConfiguration(), this.parent.MetaData.Clone(), new[] { clonedFrame });
         }
 
+        /// <summary>
+        /// Creates a new <seealso cref="ImageFrame{TPixel}" /> and appends it to the end of the collection.
+        /// </summary>
+        /// <returns>
+        /// The new <see cref="ImageFrame{TPixel}" />.
+        /// </returns>
+        public ImageFrame<TPixel> CreateFrame()
+        {
+            return this.CreateFrame(default);
+        }
+
         /// <summary>
         /// Creates a new <seealso cref="ImageFrame{TPixel}" /> and appends it to the end of the collection.
         /// </summary>
         /// <param name="backgroundColor">The background color to initialize the pixels with.</param>
         /// <returns>
         /// The new <see cref="ImageFrame{TPixel}" />.
         /// </returns>
-        public ImageFrame<TPixel> CreateFrame(TPixel backgroundColor = default) {
-            var frame = new ImageFrame<TPixel>(this.parent.GetConfiguration(), this.RootFrame.Width, this.RootFrame.Height, backgroundColor);
+        public ImageFrame<TPixel> CreateFrame(TPixel backgroundColor)
+        {
+            var frame = new ImageFrame<TPixel>(
+                this.parent.GetConfiguration(),
+                this.RootFrame.Width,
+                this.RootFrame.Height,
+                backgroundColor);
             this.frames.Add(frame);
             return frame;
         }

tests/ImageSharp.Tests/Image/ImageFramesCollectionTests.cs

@@ -225,10 +225,21 @@ public void ExtractFrame<TPixel>(TestImageProvider<TPixel> provider)
         }
 
         [Fact]
-        public void CreateFrame()
+        public void CreateFrame_Default()
         {
             this.image.Frames.CreateFrame();
+
+            Assert.Equal(2, this.image.Frames.Count);
+            this.image.Frames[1].ComparePixelBufferTo(default(Rgba32));
+        }
+
+        [Fact]
+        public void CreateFrame_CustomFillColor()
+        {
+            this.image.Frames.CreateFrame(Rgba32.HotPink);
+
             Assert.Equal(2, this.image.Frames.Count);
+            this.image.Frames[1].ComparePixelBufferTo(Rgba32.HotPink);
         }
 
         [Fact]

tests/ImageSharp.Tests/Image/ImageTests.cs

@@ -13,10 +13,60 @@ namespace SixLabors.ImageSharp.Tests
     /// <summary>
     /// Tests the <see cref="Image"/> class.
     /// </summary>
-    public class ImageTests : FileTestBase
+    public class ImageTests
     {
+        public class Constructor
+        {
+            [Fact]
+            public void Width_Height()
+            {
+                using (var image = new Image<Rgba32>(11, 23))
+                {
+                    Assert.Equal(11, image.Width);
+                    Assert.Equal(23, image.Height);
+                    Assert.Equal(11*23, image.GetPixelSpan().Length);
+                    image.ComparePixelBufferTo(default(Rgba32));
+
+                    Assert.Equal(Configuration.Default, image.GetConfiguration());
+                }
+            }
+
+            [Fact]
+            public void Configuration_Width_Height()
+            {
+                Configuration configuration = Configuration.Default.ShallowCopy();
+
+                using (var image = new Image<Rgba32>(configuration, 11, 23))
+                {
+                    Assert.Equal(11, image.Width);
+                    Assert.Equal(23, image.Height);
+                    Assert.Equal(11 * 23, image.GetPixelSpan().Length);
+                    image.ComparePixelBufferTo(default(Rgba32));
+
+                    Assert.Equal(configuration, image.GetConfiguration());
+                }
+            }
+
+            [Fact]
+            public void Configuration_Width_Height_BackroundColor()
+            {
+                Configuration configuration = Configuration.Default.ShallowCopy();
+                Rgba32 color = Rgba32.Aquamarine;
+
+                using (var image = new Image<Rgba32>(configuration, 11, 23, color))
+                {
+                    Assert.Equal(11, image.Width);
+                    Assert.Equal(23, image.Height);
+                    Assert.Equal(11 * 23, image.GetPixelSpan().Length);
+                    image.ComparePixelBufferTo(color);
+
+                    Assert.Equal(configuration, image.GetConfiguration());
+                }
+            }
+        }
+
         [Fact]
-        public void ConstructorByteArray()
+        public void Load_ByteArray()
         {
             Assert.Throws<ArgumentNullException>(() =>
             {
@@ -32,7 +82,7 @@ public void ConstructorByteArray()
         }
 
         [Fact]
-        public void ConstructorFileSystem()
+        public void Load_FileSystemPath()
         {
             TestFile file = TestFile.Create(TestImages.Bmp.Car);
             using (Image<Rgba32> image = Image.Load<Rgba32>(file.FullPath))
@@ -43,7 +93,7 @@ public void ConstructorFileSystem()
         }
 
         [Fact]
-        public void ConstructorFileSystem_FileNotFound()
+        public void Load_FileSystemPath_FileNotFound()
         {
             System.IO.FileNotFoundException ex = Assert.Throws<System.IO.FileNotFoundException>(
                 () =>
@@ -53,7 +103,7 @@ public void ConstructorFileSystem_FileNotFound()
         }
 
         [Fact]
-        public void ConstructorFileSystem_NullPath()
+        public void Load_FileSystemPath_NullPath()
         {
             ArgumentNullException ex = Assert.Throws<ArgumentNullException>(
                 () =>

tests/ImageSharp.Tests/TestUtilities/TestImageExtensions.cs

@@ -361,14 +361,25 @@ public static Image<TPixel> ComparePixelBufferTo<TPixel>(
         public static Image<TPixel> ComparePixelBufferTo<TPixel>(this Image<TPixel> image, TPixel expectedPixel)
             where TPixel : struct, IPixel<TPixel>
         {
-            Span<TPixel> actualPixels = image.GetPixelSpan();
+            foreach (ImageFrame<TPixel> imageFrame in image.Frames)
+            {
+                imageFrame.ComparePixelBufferTo(expectedPixel);
+            }
+            
+            return image;
+        }
+
+        public static ImageFrame<TPixel> ComparePixelBufferTo<TPixel>(this ImageFrame<TPixel> imageFrame, TPixel expectedPixel)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            Span<TPixel> actualPixels = imageFrame.GetPixelSpan();
 
             for (int i = 0; i < actualPixels.Length; i++)
             {
                 Assert.True(expectedPixel.Equals(actualPixels[i]), $"Pixels are different on position {i}!");
             }
 
-            return image;
+            return imageFrame;
         }
 
         public static ImageFrame<TPixel> ComparePixelBufferTo<TPixel>(