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