Fix #91: Add support for mocking variadic functions.

Alright, I can add a skip. I wasn't sure what the policy was in this repo since I didn't see skips anywhere else. For instance, when you added support for callable typehints, I noticed that you [added branches in the specs](https://github.com/phpspec/prophecy/commit/9744b84e487fac1a500f21de70eb83518d374a30) instead of maintaining version-specific specs.