FieldDoesNotExist exception when key_fields=['pk']

[cfytrok]

Please add test case, when key_fields=['pk']. bulk_sync.
get_field('pk') raises FieldDoesNotExist

django-bulk-sync/bulk_sync/__init__.py

Line 87 in 8b4dc96

for field in (db_class._meta.get_field(k) for k in key_fields)