Test case: overriding restore_object on a ModelSerializer causes the validation for model fields to be skipped

Implementing a `ModelSerializer` with an overriden `restore_object` method causes that serializer to skip the validation for model fields. Validation without `restore_object` on the `ModelSerializer` subclass works just fine.

This error was introduced between 2.1.6 and 2.1.7 (I tested individual versions), but I cannot find the source of the error.

The pull request contains a simple test case for this scenario: A `ModelSerializer` with an override for `restore_object`, taken from other Serializers in the test case file. No fix is attached.