Add tests for unique_with constraint

[Vayel]

A test for #239. It fails.

The field with the unique_with parameter has a UniqueValidator in the serializer whereas it shouldn't.

TestSerializer():
    id = ObjectIdField(read_only=True)
    ref1 = ReferenceField(queryset=ReferencedDoc.objects, required=True, validators=[<UniqueValidator(queryset=ReferencedDoc.objects)>])
    ref2 = ReferenceField(queryset=OtherReferencedDoc.objects, required=True)
    class Meta:
        validators = [<UniqueTogetherValidator(queryset=ReferencingDocWithUniqueWithField.objects, fields=('ref1', 'ref2'))>]

Due to the UniqueValidator on ref1, Mongoengine tells use that ReferencedDoc has no field ref1.