Correctly invoke handler on -[FBURLConnection cancel]

Problem:
Client's completion block is not called on a FBURLConnection when that
connection is terminated by -cancel such as when a timeout fires in the client's code.

Cause:
-cancel sets *self.handler* to nil before calling invokeHandler. The
cancel method retains a copy of *handler* before calling
invokeHandler.

While invokeHandler uses the *handler* parameter for calling, it was
checking *self.handler* for non-nil before doing so.

All other calls to invokeHandler are made with *self.handler* as the
parameter so the issue wasn't apparent in 'normal' use.