Allowed async functions to be passed into spy#callFake

src/core/SpyStrategy.js

@@ -88,7 +88,7 @@ getJasmineRequireObj().SpyStrategy = function(j$) {
      * @param {Function} fn The function to invoke with the passed parameters.
      */
     this.callFake = function(fn) {
-      if(!j$.isFunction_(fn)) {
+      if(!(j$.isFunction_(fn) || j$.isAsyncFunction_(fn))) {
         throw new Error('Argument passed to callFake should be a function, got ' + fn);
       }
       plan = fn;