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