Added a relationship deletion unit test
Added a relationship deletion unit test

To delete a relationship, the corresponding value
is being overriden with None. Eg:
{
    'data': {
        'type': 'posts',
        'relationships': {
            'blog': {
                'data': None
            }
        }
    }
}

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