Python: Fix Flask jsonify XSS regression

The reason the result was found before, is that `jsonify(data)` was
modeled as TWO separate subclasses of `Http::Server::HttpResponse`, one
because of the implicit construction in return
(FlaskRouteHandlerReturn), and one from the `jsonify` call
(FlaskJsonifyCall). Due to the QL evaluation, we got a combination from
the two, meaning mime-type from FlaskRouteHandlerReturn and body from
FlaskJsonifyCall...

python/ql/lib/semmle/python/frameworks/Flask.qll

@@ -453,7 +453,8 @@ module Flask {
     FlaskRouteHandlerReturn() {
       exists(Function routeHandler |
         routeHandler = any(FlaskRouteSetup rs).getARequestHandler() and
-        node = routeHandler.getAReturnValueFlowNode()
+        node = routeHandler.getAReturnValueFlowNode() and
+        not this instanceof Flask::Response::InstanceSource
       )
     }