def each_header   #:yield: +key+, +value+
      block_given? or return enum_for(__method__)
      @header.each do |k,va|
        yield k, va.join(', ')
      end
    end
def each_header   #:yield: +key+, +value+
      block_given? or return enum_for(__method__)
      @header.each do |k,va|
        yield k, va.join(', ')
      end
    end
Iterates for each header names and values.