def []=(key, val)
      unless val
        @header.delete key.downcase
        return val
      end
      @header[key.downcase] = [val]
    end
def []=(key, val)
      unless val
        @header.delete key.downcase
        return val
      end
      @header[key.downcase] = [val]
    end
Sets the header field corresponding to the case-insensitive key.