Fix copyAssetsFileIOS's image resizing option to resize to exact width and height

[byeokim]

copyAssetsFileIOS accepts width and height and resizes image when copying from Camera Roll.

copyAssetsFileIOS(imageUri, destPath, width, height, scale, compression, resizeMode)

However currently it doesn't respect width and height, and copied image may or may not have width and height that match width and height. That's because copyAssetsFileIOS uses PHImageRequestOptionsResizeModeFast option which makes requestImageForAsset method efficiently resize the image to a size similar to, or slightly larger than, the target size.

react-native-fs/RNFSManager.m

Lines 808 to 816 in f2f8f4a

	BOOL useMaximumSize = CGSizeEqualToSize(size, CGSizeZero);
	CGSize targetSize;
	if (useMaximumSize) {
	targetSize = PHImageManagerMaximumSize;
	imageOptions.resizeMode = PHImageRequestOptionsResizeModeNone;
	} else {
	targetSize = CGSizeApplyAffineTransform(size, CGAffineTransformMakeScale(scale, scale));
	imageOptions.resizeMode = PHImageRequestOptionsResizeModeFast;
	}

This pull request changes the option from PHImageRequestOptionsResizeModeFast to PHImageRequestOptionsResizeModeExact which makes requestImageForAsset method resize the image to match the target size, i.e. width and height, exactly.

[itinance]

Wow. Thank you!