code: public void addField(String name, String value)
          throws IllegalStateException
      {
          if (_state!=__MSG_EDITABLE)
              return;
          _header.add(name,value);
      }
docs: Add to 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 value to add to the field
  @exception IllegalStateException Not editable or sending 1.1
  with trailers