[core] Restore existing response body

Until #2721 lands we still need this.

platform/default/http_request_curl.cpp

@@ -532,6 +532,8 @@ void HTTPCURLRequest::handleResult(CURLcode code) {
             response->notModified = true;
 
             if (existingResponse) {
+                response->data = existingResponse->data;
+
                 if (response->expires == Seconds::zero()) {
                     response->expires = existingResponse->expires;
                 }