Add getTileUrlFunction in ol.source.TileImage

src/ol/source/tileimagesource.js

@@ -119,6 +119,15 @@ ol.source.TileImage.prototype.getTile =
 };
 
 
+/**
+ * @return {ol.TileUrlFunctionType} TileUrlFunction
+ * @todo stability experimental
+ */
+ol.source.TileImage.prototype.getTileUrlFunction = function() {
+  return this.tileUrlFunction;
+};
+
+
 /**
  * @param {ol.TileUrlFunctionType} tileUrlFunction Tile URL function.
  */