code: public String toUrlEncodedString() {
          QueryStringEncoder encoder = new QueryStringEncoder("");
          for (Map.Entry<String, String> entry : formValues.entrySet()) {
              encoder.addParam(entry.getKey(), entry.getValue());
          }
          StringBuilder ues = new StringBuilder(encoder.toString());
          if (ues.length() > 0 && ues.charAt(0) == '?') {
              ues.deleteCharAt(0);
          }
          return ues.toString();
      }
docs: Encode the {@link Form} using the "application/x-www-form-urlencoded" Content-Type.
  Each form parameter is represented as "key=value" where both key and value are url-encoded.
  
  @return the url-encoded representation of the form.