Fixed the Collaborator when it is a ObjectProphecy.

This came up when updating specs for prophecy which mocks ObjectProphecy. It was actually calling the method on the ObjectProphecy instead of returning a MethodProphecy.
Fixed unwrapper for arguments implementing the WrapperInterface.

Wrapped object is already revealed.
Merge pull request #116 from jakzal/collaborator-fix

Fixed the Collaborator and Unwrapper