Replaced MHD_RESPMEM_PERSISTENT usage in examples and code

New functions do not produce compiler warnings.

src/testcurl/https/test_https_get.c

@@ -120,9 +120,7 @@ ahc_empty (void *cls,
     return MHD_YES;
   }
   *req_cls = NULL;
-  response = MHD_create_response_from_buffer (0,
-                                              NULL,
-                                              MHD_RESPMEM_PERSISTENT);
+  response = MHD_create_response_empty (MHD_RF_NONE);
   ret = MHD_queue_response (connection,
                             MHD_HTTP_OK,
                             response);