fix DateTime json encode serialisation

This fix the problem #1412 
Is not complete, because I trying to find a way for the user customise the *jsonEncodable* but I can't find a way that do not changes the signature for Function request = (HttpResponseConfig config). If I inject the configuration it can't be  accessed in the method because *this*  is not in the scope. 
One solution should create a static factory in other class.
This should probably be `new DateTime.utc(...)` to work whatever the local TZ is.