Factor out and/or remove UTC+0 stripping code

For datetime and time objects in the UTC+0 timezone, we currently do this:

``` python
if r.endswith('+00:00'):
r = r[:-6] + 'Z'
```

This should either be
- factored out into a separate function
- removed entirely if there's no need to strip this. Arguably it's actually bad to strip it, because UTC+0 timezone != no timezone.

This timezone stripping originally came from core Django (https://github.com/django/django/blob/master/django/core/serializers/json.py#L94-L95). Why does Django does this in the first place? There has to be a reason.

Locate the encoder code containing the shown `r.endswith('+00:00')` branch, then compare its behavior with the linked Django serializer implementation and the UTC semantics described here. Decide whether the `Z` conversion is required, and finish by making the chosen behavior explicit and consistent for datetime and time objects.

Written by the indexing model from the issue text.