code: public String removeField(String name)
          throws IllegalStateException
      {
          if (_state!=__MSG_EDITABLE)
              return null;
          return _header.remove(name);
      }
docs: Remove a field.
  If the message is editable, then a header field is removed. Otherwise
  if the message is sending and a HTTP/1.1 version, then a trailer
  field is removed.
  @param name Name of field
  @return Old value of field