code: public Request userAgent(String userAgentValue){
          final String userAgent = RequestHeaderFields.USER_AGENT.getName();
          String agent = headers.get(userAgent);
          if (agent == null){
              agent = userAgentValue;
          } else {
              agent = agent + " " + userAgentValue;
          }
          return this.setHeader(userAgent, agent);
      }
docs: sets the user agent header for the request, if exists then appends
  @param userAgentValue   : header field value to add
  @return : Request Object with userAgent header value set