change to allow cancellation of requests

I have modified the send methods in the Method and JsonpMethod classes to return the created Request or JsonpRequest instances so the generated proxy class can also return them to the client.

I had to use Object as the return type in the Method class because Request and JsonpRequest don't have anything in common (no base class and no interfaces).

The generator validates the method return type and casts the object according to its type, so the client will in fact get one of the two request objects.

By having access to the request objects the client can cancel the pending requests.

I have tried to create tests for this change, but since the tests I have modified are not being executed, I though it would not make sense to try to fix this in this commit.