When a `pk` was used as a relationship value a json error was thrown
When a `pk` was used as a relationship value a json error was thrown

For this (incorrect) payload the related field would `json.loads`
the `bar` related attribute:
``` py
{
    "data": {
        "type": "foo",
        "attributes": {
            "bar": "1",
        }
    }
}
```
a json ValueError would be thrown instead of the more helpful
"Expected resource identifier object, received unicode"

scottfisk/django-rest-framework-json-api,django-json-api/django-rest-framework-json-api,leo-naeka/django-rest-framework-json-api,abdulhaq-e/django-rest-framework-json-api,lukaslundgren/django-rest-framework-json-api,Instawork/django-rest-framework-json-api,martinmaillard/django-rest-framework-json-api,pombredanne/django-rest-framework-json-api,django-json-api/django-rest-framework-json-api,schtibe/django-rest-framework-json-api,django-json-api/rest_framework_ember,leo-naeka/rest_framework_ember