assign_default_content_type_and_charset!

# File actionpack/lib/action_dispatch/http/response.rb, line 340
    def assign_default_content_type_and_charset!(headers)
      return if headers[CONTENT_TYPE].present?

      @content_type ||= Mime::HTML
      @charset      ||= self.class.default_charset unless @charset == false

      type = @content_type.to_s.dup
      type << "; charset=#{@charset}" if append_charset?

      headers[CONTENT_TYPE] = type
    end