Fix small bug in image2 plugin

  * Fix missing check of the query string presence in the src of the
    image before adding a random query string value for cache purpose
  * fixes ckeditor#3394

plugins/image2/dialogs/image2.js

@@ -121,9 +121,10 @@ CKEDITOR.dialog.add( 'image2', function( editor ) {
 			addListener( 'abort', function() {
 				callback( null );
 			} );
-
+
+			var queryStringSeparator = (src.indexOf('?') > -1) ? '&' : '?';
 			image.setAttribute( 'src',
-				( config.baseHref || '' ) + src + '?' + Math.random().toString( 16 ).substring( 2 ) );
+				( config.baseHref || '' ) + src + queryStringSeparator + Math.random().toString( 16 ).substring( 2 ) );
 		};
 	}