Added breaking test for JSONP support via Rack::JSONP (core issue is likely to be more generic and centered around middleware/formatter etc.).

Added breaking test for JSONP support via Rack::JSONP (core issue is likely to be more generic and centered around middleware/formatter etc.).

Related discussion:

https://github.com/intridea/grape/pull/346
https://github.com/intridea/grape/issues/345