def fetch(key, *args, &block)   #:yield: +key+
      a = @header.fetch(key.downcase, *args, &block)
      a.kind_of?(Array) ? a.join(', ') : a
    end
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 IndexErrror if there's no header field named +key+
 See Hash#fetch