    # File lib/action_mailer/vendor/tmail-1.2.3/tmail/mail.rb, line 270
        def []=( key, val )
          dkey = key.downcase

          if val.nil?
            @header.delete dkey
            return nil
          end

          case val
          when String
            header = new_hf(key, val)
          when HeaderField
            ;
          when Array
            ALLOW_MULTIPLE.include? dkey or
                    raise ArgumentError, "#{key}: Header must not be multiple"
            @header[dkey] = val
            return val
          else
            header = new_hf(key, val.to_s)
          end
          if ALLOW_MULTIPLE.include? dkey
            (@header[dkey] ||= []).push header
          else
            @header[dkey] = header
          end

          val
        end