Customizer: Allow to skip cropping header images if image width is smaller than or equal to theme width.

Add tests.

see #27936 for trunk.

git-svn-id: https://develop.svn.wordpress.org/trunk@28219 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/js/customize-models.js

@@ -113,6 +113,10 @@
 				return false;
 			}
 
+			if (this.get('imageWidth') <= this.get('themeWidth')) {
+				return false;
+			}
+
 			return true;
 		}
 	});