When a method doesn't accept keyword arguments.

In this scenario keyword or Hash-type arguments are assigned as a single
Hash to the last argument without any warnings and strict keyword
matching should not have any effect.

This is an exploratory spike on fixing #593.

* This has highlighted a significant problem with partial mocks in #532.
  The method obtained from the responder is the stub method defined by
  Mocha and not the original. This effectively makes it useless!

* I'm not sure the method_accepts_keyword_arguments? belongs on
  Invocation, but that's the most convenient place for now. It feels as
  if we need to have a bit of a sort out of where various things live
  and perhaps introduce some new classes to make things clearer.

* We might want to think ahead a bit at what we want to do in #149 to
  decide the best way to go about this.

* I'm not sure it's sensible to re-use the Equals matcher; we could
  instead parameterize PositionalOrKeywordHash, although the logic in
  there is already quite complex. Conversely if this is a good approach,
  it might make more sense to do something similar when creating a hash
  matcher for a non-last parameter to further simplify the code.

* I haven't yet introduced any acceptance tests for this and I suspect
  there might be some edge cases yet to come out of the woodwork. In
  particular, I think it's worth exhaustively working through the
  various references mentioned in this comment [1].

[1]: #593 (comment)