RequestContext::create should call onUnset callback

RequestContext::create should call onUnset callback

Summary: melaniesubbiah introduced onSet / onUnset callbacks on RequestData in D3604948; we need unset() to be called when an RC is overriden with RequestContext::create() so that things work as expected. Also, change the order of calling onSet / onUnset - from RequestData perspective, it shouldn't look like there are two contexts set at the same time

Reviewed By: palmtenor

Differential Revision: D3667017

fbshipit-source-id: b9bfb858fe65ffb11de8e6d6f13b8f4cf6266bc9