Prioritize env API_FORMAT over Rack::CONTENT_TYPE header

[trialize]

As per #2171

Fix the Order of Conditions in fetch_formatter to Correctly Use Custom Formatters

Issue:

In Grape::Middleware::Formatter.fetch_formatter, the current order of conditions causes custom formatters specified in env[Grape::Env::API_FORMAT] to be ignored in favor of the default formatter associated with the Content-Type header.

Original Code:

def fetch_formatter(headers, options)
  api_format = mime_types[headers[Grape::Http::Headers::CONTENT_TYPE]] || env[Grape::Env::API_FORMAT]
  Grape::Formatter.formatter_for(api_format, **options)
end

Problem:

This code prioritizes the Content-Type header over the env[Grape::Env::API_FORMAT]. As a result, even if a custom formatter is explicitly set in the environment, it may be overridden by the default formatter determined by the Content-Type.

Solution:

Swap the order of conditions so that env[Grape::Env::API_FORMAT] takes precedence:

def fetch_formatter(headers, options)
  api_format = env[Grape::Env::API_FORMAT] || mime_types[headers[Grape::Http::Headers::CONTENT_TYPE]]
  Grape::Formatter.formatter_for(api_format, **options)
end

Rationale:

By giving priority to env[Grape::Env::API_FORMAT], we ensure that custom formatters are correctly recognized and applied, allowing for more flexible and accurate response formatting.

Example:

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

# Use the custom formatter within an endpoint
get do
  env['api.format'] = :raw_json
  body <<-HERE
  {
    "glossary": {
      "title": "example glossary",
      "GlossDiv": {
        "title": "S",
        "GlossList": {
          "GlossEntry": {
            "ID": "SGML",
            "SortAs": "SGML",
            "GlossTerm": "Standard Generalized Markup Language",
            "Acronym": "SGML",
            "Abbrev": "ISO 8879:1986",
            "GlossDef": {
              "para": "A meta-markup language, used to create markup languages such as DocBook.",
              "GlossSeeAlso": ["GML", "XML"]
            },
            "GlossSee": "markup"
          }
        }
      }
    }
  }
  HERE
end

Before the Fix:

The custom formatter :raw_json is not applied. Instead, Grape uses its default JSON formatter.

After the Fix:

The custom formatter :raw_json is correctly applied, and the response body is returned exactly as intended.

[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?