Add response status matcher

Allow for more fluid response status checks, supporting both symbols and numbers:

```ruby
expect(response).to have_status(:success)
is_expected.to respond_with_status(404)
expect(response).to return_status(:error)
```

This is already taken care of by [`assert_response`](http://apidock.com/rails/ActionDispatch/Assertions/ResponseAssertions/assert_response). So this can just be a thin wrapper around that.