code: public void setField(String name, List value)
      {
          if (_state!=__MSG_EDITABLE)
              return;
          _header.put(name,value);
      }
docs: Set a multi-value field value.
  If the message is editable, then a header field is set. Otherwise
  if the meesage is sending and a HTTP/1.1 version, then a trailer
  field is set.
  @param name Name of field
  @param value New values of field