Prioritize env API_FORMAT over Rack::CONTENT_TYPE header

[trialize]

As per #2171

UPDATE: Proposed solution is not appropriate and requires further investigation

[grape-bot]

	2 Warnings
⚠️	There're library changes, but not tests. That's OK as long as you're refactoring existing code.
⚠️	Unless you're refactoring existing code or improving documentation, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#2505](https://github.com/ruby-grape/grape/pull/2505): Prioritize env api_format over rack::content_type header - [@trialize](https://github.com/trialize).

Generated by 🚫 Danger

[dblock]

Looks reasonable.

This needs a test, a CHANGELOG entry, a paragraph in UPGRADING, and some documentation about order of formatters in README, please.

This is a breaking change, but also likely a bug. If no existing tests fail we are good with the above.

I see this test fail.

  1) Grape::API.format :json can be overwritten with an explicit content type
     Failure/Error: expect(last_response.body).to eq({ meaning_of_life: 42 }.to_s)

       expected: #<Encoding:US-ASCII> "{:meaning_of_life=>42}"
            got: #<Encoding:ASCII-8BIT> "\"{:meaning_of_life=>42}\""

       (compared using ==)
     # ./spec/grape/api_spec.rb:4101:in `block (4 levels) in <top (required)>'

Let's understand whether the test is just wrong?

[trialize]

Closed due to lack of time to address the case according to CONTRIBUTING requirements.

Following code

get do
  content_type 'application/json'
  env['api.format'] = :binary
  '{ "meaning_of_life": 42 }'
end

Still would apply json formatter and produce "{ \"meaning_of_life\": 42 }" which is a bug.

As a workaround, for those having the need, following could be used on separate resources.

module API
  class myAPI < Grape::API
    resource :raw_json do

    content_type :raw_json, 'application/json'
    formatter :raw_json, ->(object, _env) { object }

    get do
      '{ "myRawJson": 123 }'
    end

  end
end

(P.S. This will override Grape's default JSON formatter, meaning you'd lose it one other endpoints as well)

[dblock]

Hopefully someone can finish this, please!