def fetch(key, *args, &block)   #:yield: +key+
    a = @header.fetch(key.downcase, *args, &block)
    a.kind_of?(Array) ? a.join(', ') : a
  end
Returns the header field corresponding to the case-insensitive key.
Returns the default value +args+, or the result of the block, or
raises an IndexError if there's no header field named +key+
See Hash#fetch