code: public static void addDefaultHeader(String key, String value) {
  		if (isNotBlank(key) && isNotBlank(value)) {
  			if (defaultHeader == null) {
  				defaultHeader = new HashMap<String, String>();
  			}
  			defaultHeader.put(key, value);
  		}
  	}
docs: Added default headers will always be send, can still be overriden using the builder.