Avoid allocating StringResponse when no response filters are active

AjaxRequestHandler#respond currently always allocates a new StringResponse:

final StringResponse bodyResponse = new StringResponse();
update.writeTo(bodyResponse, encoding);
CharSequence filteredResponse = invokeResponseFilters(bodyResponse);
response.write(filteredResponse);

Wrapping the response in a StringResponse is necessary for applying ResponseFilters that potentially modify the response.

The thing is this: By default, a Wicket application running in deployment mode, does not have any response filters active. So in that case, we can avoid all of this expensive buffering.