Run after filters before verifying there are expectations

Currently, spec gems such as WebStub and motion-stump have difficulty expressing mock expectations, because these sorts of mocks usually would end up in an `after` block registered by the library. However, Bacon expects `after` blocks to be used for clean-up only. Because `after` blocks are not allowed to contain expectations, users have to clutter their code with bogus expectations like `1.should == 1` in order to get the tests to pass.

This pull request simply runs the after filters before verifying at least one expectation is set, allowing `after` blocks to contain expectations.