Merge pull request #305 from Unity-Technologies/fix_double_tap_issue

fix double tap bug

com.unity.uiwidgets/Runtime/gestures/multitap.cs

         void _checkUp(int buttons) {
             D.assert(buttons == gesture_.kPrimaryButton);
             if (onDoubleTap != null) {
-                invokeCallback("onDoubleTap",() => onDoubleTap);
+                invokeCallback("onDoubleTap",() => onDoubleTap());
             }
         }
 

com.unity.uiwidgets/Runtime/gestures/recognizer.cs

                     return true;
                 });
 
-                
+                callback();
             }
             catch (Exception ex) {
                 IEnumerable<DiagnosticsNode> infoCollector() {