bugfix optiongroupserializer missing

[svandeneertwegh]

Added: option_group = AttributeOptionGroupSerializer()

to:

class OptionSerializer(OscarHyperlinkedModelSerializer):

in file: oscarapi/serializers/product.py

[svandeneertwegh]

This fixes the problem mentioned at #324 !

[svandeneertwegh]

@specialunderwear when this is not applied i get errors on admin endpoints when i set: OSCARAPI_BLOCK_ADMIN_API_ACCESS = False