code: public OkRequest<T> param(final String key, final String value) {
          StringBuilder urlBuilder = new StringBuilder(getUrl());
          if (getUrl().contains("?")) {
              urlBuilder.append("&");
          } else {
              urlBuilder.append("?");
          }
          urlBuilder.append(key);
          urlBuilder.append("=");
          urlBuilder.append(value);
          mRequestUrl = urlBuilder.toString();
          return this;
      }
docs: Write the value to url params
  
  @param key   key
  @param value value
  @return this request