Trim the last EOL in AppendingTransformer processing

This is useful for supporting the custom line separator in the future. In that case we can handle `resources/application.yml` files, that are separated by `---`.
I did the same patch for Gradle Shadow plugin in GradleUp/shadow@8300f91.

Author: Goooler

src/main/java/org/apache/maven/plugins/shade/resource/AppendingTransformer.java

@@ -46,8 +46,11 @@ public boolean canTransformResource(String r) {
     @Override
     public void processResource(String resource, InputStream is, List<Relocator> relocators, long time)
             throws IOException {
+        if (data.size() > 0) {
+            // Append the EOL before the new content to ensure the EOL is not at the end of the file.
+            data.write('\n');
+        }
         IOUtil.copy(is, data);
-        data.write('\n');
         if (time > this.time) {
             this.time = time;
         }

src/test/java/org/apache/maven/plugins/shade/resource/AppendingTransformerTest.java

@@ -18,11 +18,19 @@
  */
 package org.apache.maven.plugins.shade.resource;
 
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.Collections;
 import java.util.Locale;
+import java.util.jar.JarInputStream;
+import java.util.jar.JarOutputStream;
 
 import org.junit.Before;
 import org.junit.Test;
 
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
@@ -56,4 +64,37 @@ public void testCanTransformResource() {
         assertTrue(transformer.canTransformResource("ABCDEFGHIJKLMNOPQRSTUVWXYZ"));
         assertFalse(transformer.canTransformResource("META-INF/MANIFEST.MF"));
     }
+
+    @Test
+    public void testProcessResource() throws IOException {
+        transformer.resource = "test-resource";
+        String firstLine = "first line";
+        String secondLine = "second line";
+        InputStream firstIs = new ByteArrayInputStream(firstLine.getBytes());
+        InputStream secondIs = new ByteArrayInputStream(secondLine.getBytes());
+
+        transformer.processResource("", firstIs, Collections.emptyList());
+        transformer.processResource("", secondIs, Collections.emptyList());
+
+        final ByteArrayOutputStream out = new ByteArrayOutputStream();
+        try (final JarOutputStream jarOutputStream = new JarOutputStream(out)) {
+            transformer.modifyOutputStream(jarOutputStream);
+        }
+
+        try (final JarInputStream jis = new JarInputStream(new ByteArrayInputStream(out.toByteArray()))) {
+            assertEquals("test-resource", jis.getNextJarEntry().getName());
+            String result = read(jis);
+            assertEquals(firstLine + "\n" + secondLine, result);
+        }
+    }
+
+    private String read(final JarInputStream jar) throws IOException {
+        final StringBuilder builder = new StringBuilder();
+        final byte[] buffer = new byte[512];
+        int read;
+        while ((read = jar.read(buffer)) >= 0) {
+            builder.append(new String(buffer, 0, read));
+        }
+        return builder.toString();
+    }
 }