Grape::Entity leaks @options and other internals when presented without a formatter

[dblock]

When presented without a formatter a Grape::Entity gets serialized with all its internals, including @options. This doesn't appear to leak any sensitive information most of the time, but YMMV as the rack app, session, request objects and ActiveRecord internals come along.

It's a nasty side effect of adding something like content_type :csv, 'text/csv' and not having a general formatter :csv, ... that catches everything and calls, for example, to_csv. Entities by default support to_xml and to_json, but nothing else.

Complete example in https://gist.github.com/dblock/5b49741468efd5b6a7d6828167d47a65.