code: public Request single(final CharSequence name) throws HttpException {
          final Iterator<Request> parts = this.part(name).iterator();
          if (!parts.hasNext()) {
              throw new HttpException(
                  HttpURLConnection.HTTP_BAD_REQUEST,
                  String.format(
                      "form param \"%s\" is mandatory", name
                  )
              );
          }
          return parts.next();
      }
docs: Get single part.
  @param name Name of the part to get
  @return Part
  @throws HttpException If fails