docs: protected void addProxyConnectionHeader(HttpState state,
                                              HttpConnection conn)
      throws IOException, HttpException {
          LOG.trace("enter HttpMethodBase.addProxyConnectionHeader("
                    + "HttpState, HttpConnection)");
          if (!conn.isTransparent()) {
          	if (getRequestHeader("Proxy-Connection") == null) {
                  addRequestHeader("Proxy-Connection", "Keep-Alive");
          	}
          }
      }