Add spec demonstrating odd `method_missing` behavior.

I ran into this in some rspec-mocks specs for rspec/rspec-mocks#23 but traced it back to rspec-expectations.  This isn't meant to be merged yet (we need to solve the issue first) but I wanted to at least document the issue with a failing spec.

It's very, very odd...I put comments in the example that explain it in detail.

Anyone want to take a stab at troubleshooting this?