[core] Restore existing response body

Until #2721 lands we still need this.

platform/android/src/http_request_android.cpp

@@ -209,6 +209,8 @@ void HTTPAndroidRequest::onResponse(int code, std::string message, std::string e
         response->notModified = true;
 
         if (existingResponse) {
+            response->data = existingResponse->data;
+
             if (response->expires == Seconds::zero()) {
                 response->expires = existingResponse->expires;
             }