def write_header(key, value)
      # @see http://stomp.github.io/stomp-specification-1.2.html#Repeated_Header_Entries
      key = decode_header(key)
      @headers[key] = decode_header(value) unless @headers.has_key?(key)
    end
def write_header(key, value)
      # @see http://stomp.github.io/stomp-specification-1.2.html#Repeated_Header_Entries
      key = decode_header(key)
      @headers[key] = decode_header(value) unless @headers.has_key?(key)
    end
Write a single header to this frame.

 @param [String] key
 @param [String] value