Supports `has_attribute?` for `mock_model` doubles

Fixes #956 (Rails 4.0.4 interop)

Once this lands in 2.14, we should:
- [ ] Rebase/merge to rspec-activemodel-mocks (cc: @thomas-holmes)
- [ ] Rebase/merge to 2-99-maintenance
- [ ] Release a new 2.14 version to rubygems

/cc: @JonRowe, @huoxito