Add failing test for ListField schema generation

The `ListField` was generating a schema that contained `type=None`
when a `ChoiceField` was the child, since we are not currently able
to introspect the type of a `ChoiceField`.

Author: kevin-brown

tests/schemas/test_openapi.py

@@ -52,6 +52,8 @@ def test_list_field_mapping(self):
             (serializers.ListField(child=serializers.CharField()), {'items': {'type': 'string'}, 'type': 'array'}),
             (serializers.ListField(child=serializers.IntegerField(max_value=4294967295)),
              {'items': {'type': 'integer', 'format': 'int64'}, 'type': 'array'}),
+            (serializers.ListField(child=serializers.ChoiceField(choices=[('a', 'Choice A'), ('b', 'Choice B')])),
+             {'items': {'enum': ['a', 'b']}, 'type': 'array'}),
             (serializers.IntegerField(min_value=2147483648),
              {'type': 'integer', 'minimum': 2147483648, 'format': 'int64'}),
         ]