code: public String single(final CharSequence name) throws IOException {
          final Iterator<String> params = this.param(name).iterator();
          if (!params.hasNext()) {
              throw new HttpException(
                  HttpURLConnection.HTTP_BAD_REQUEST,
                  String.format(
                      "form param \"%s\" is mandatory", name
                  )
              );
          }
          return params.next();
      }
docs: Get single param or throw HTTP exception.
  @param name Name of query param
  @return Value of it
  @throws IOException If fails