RablEngine to clear instance members of scope on render

[gautampunhani]

Was using Rabl with Sinatra which uses template caching. The RablTemplates intitialise RablEngine objects which copies the instance members of scope object. So, if on second call none of my instance members on scope object are set, it will still use the instance members of my first call.

With each render call the instance members of engine can be cleared first and then set from the scope object.