Remove outdated comment
Return null type format when format is not know

When requesting a controller with the following code with a unknown format:

    def my_action
      respond_to do |format|
        format.json { head :ok }
        format.any { render text: 'Default response' }
      end
    end

we should render the default response instead of raising ActionController::UnknownFormat

Fixes #14462

Conflicts:
	actionpack/CHANGELOG.md
	actionpack/test/controller/mime/respond_with_test.rb

Conflicts:
	actionpack/CHANGELOG.md