Calling declared(params) from child namespace fails to include parent namespace defined params

The test in this PR highlights the problem as described in the subject. 

The [docs](https://github.com/intridea/grape#namespace-validation-and-coercion) seem to suggest that a params block defined against `resource` in this way should apply to every method within the resource(/namespace)

Hence the expectation that a call to `declared(params)` should include the param defined against the resource.

Does this make sense? Hopefully the test does!