code: public Request ifModifiedSince(String dateSince){
          final String ifmodsin = RequestHeaderFields.IF_MODIFIED_SINCE.getName();
          return this.setHeader(ifmodsin, dateSince);
      }
docs: sets the if modified since header for the request (use with GET only)
  @param dateSince   : header field value to add
  @return  : Request Object with if_modified_since header value set