code: public B setCustomUserAgent(String customUserAgent) {
          if (customUserAgent != null && !customUserAgent.isEmpty()) {
              httpParams.customUserAgent = customUserAgent;
          }
          return self();
      }
docs: Sets the custom user agent to use for this upload request.
  Note! If you set the "User-Agent" header by using the "addHeader" method,
  that setting will be overwritten by the value set with this method.
  
  @param customUserAgent custom user agent string
  @return self instance