Add description for object to rspec support

Necessary for https://github.com/rspec/rspec-mocks/pull/819
I don't think it makes it any easier to lift to `spec_helper`.  How do you see it being easier?

Unless I'm missing something, the only advantage I see to wrapping it in a module is that you can access nested constants from within the matcher without needing the full qualification of the outer module...but you aren't referencing any constants, so the `FakeMatcher` module isn't actually doing anything here.