Update ERestJSONOutputWidget.php

Fix check for virtual attributes.

In last commit, you add this:
https://github.com/evan108108/RESTFullYii/blob/217185696a9530885ad29ccb3b6b54d4c83f24aa/starship/RestfullYii/widgets/ERestJSONOutputWidget.php#L229

However, in my model, I have created some virtual attributes that are present only if application was in REST mode. So I add the check you can see in this PR.

Tested and work!