resource urls now accept alphanumeric primary keys

I have an app with a Model imported from a table (inspectdb) with a CharField primary key. The patch supports other types as primary key with backwards compatibility.

By trying to get the object with a RESTful call, i was unable to get a object by primary key:

    /api/v1/model/my_primary_key/?format=xml

After applying this patch it works and tests passed for me.

Best Regards,
Jordi