docs: public void appendField(final String key, final String value) {
          if (builder.length() > 0) {
              builder.append("&");
          }
  
          try {
              builder.append(key).append("=").append(URLEncoder.encode(value, "UTF-8"));
          } catch (UnsupportedEncodingException e) {
              throw new AssertionError("UTF-8 encoding should always be available.");
          }
      }