Options Errors and Alias :with and :using

Should fix https://github.com/intridea/grape-entity/issues/49.

Notes:

* options are passed in two distinct places (in `merge_options` and in `with_options`) so a helper method `valid_options` covers both

* had to change an existing spec to avoid using an unknown option

* there's 6 `rubocop` offences but none of them are mine