Support different incoming to outgoing content-type

I am attempting to upgrade to v1 and have been banging my head against problems caused by #1589

We do some JSON Merge Patch – prior to #1589 that was as simple as:

if request.content_type == 'application/merge-patch+json'
  # do something different
end

Now I have to do some of this to dodge 406's

content_type :json_merge_patch, 'application/merge-patch+json'
format :json_merge_patch
formatter :json_merge_patch, Grape::Formatter::ActiveModelSerializers
error_formatter :json_merge_patch, ACME::JsonErrorFormatter
parser :json_merge_patch, Grape::Parser::Json

And this works, but now the API serves out Content-Type: application/merge-patch+json – but it should still be application/json, the request was partial but the response is full.

I expect I need to adjust the formatter to do this... but given ours is ActiveModelSerializers it's pretty abstract and I am not sure where JSON comes into it.

Is there really the need for all of this boilerplate to do this?

Start by reproducing the issue's v1 configuration with the listed parser, formatter, error formatter, and ActiveModelSerializers integration, while referencing the behavior introduced by #1589. Trace how the incoming application/merge-patch+json type determines the outgoing response type. Done means accepting merge-patch requests while responses use application/json without the current boilerplate.