code: public String setField(String name, String value)
      {
          if (_state!=__MSG_EDITABLE)
              return null;
  
          if (HttpFields.__ContentType.equalsIgnoreCase(name))
          {
              String old=_header.get(name);
              setContentType(value);
              return old;
          }
          
          return _header.put(name,value);
      }
docs: Set a field value.
  If the message is editable, then a header field is set. Otherwise
  if the message is sending and a HTTP/1.1 version, then a trailer
  field is set.
  @param name Name of field
  @param value New value of field
  @return Old value of field