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.
```(+|-)``` causes FormatException: nothing to repeat^((+|-)?\d{4,5})-?(\d\d)-?(\d\d)(?:[ T](\d\d)(?::?(\d\d)(?::?(\d\d)(.\d{1,6})?)?)? ?([zZ])?)?$, #0