code: public static void addVaryAcceptEncoding(final GZipServletResponseWrapper wrapper) {
  		Collection<String> headers = wrapper.getHeaderNames();
  
  		String varyHeader = null;
  		for (String header : headers) {
  			if (header.equals("Vary")) {
  				varyHeader = wrapper.getHeader(header);
  				break;
  			}
  		}
  
  		if (varyHeader == null) {
  			wrapper.setHeader("Vary", "Accept-Encoding");
  		} else {
  			if (!varyHeader.equals("*") && !varyHeader.contains("Accept-Encoding")) {
  				wrapper.setHeader("Vary", varyHeader + ",Accept-Encoding");
  			}
  		}
  	}
docs: Adds the Vary: Accept-Encoding header to the response if needed.
  @param wrapper response