Add validate method back to ListSerializer

Fixes #2168.
I would probably recommend we drop the call to super here, in the same way we do for `Serializer`. It gets pretty confusing otherwise. This'd also let us move the `if not isinstance(data, list)` check out of `to_internal_value` which would be nicer and would mirror the `Serializer` classes `if not isinstance(data, dict)` check that occurs.