Fix issue with model passed to Resource constructor crashes on export

[matthewhegarty]

Problem

Closes #1716

Solution

Passing the model class to the constructor is redundant.

Acceptance Criteria

• Added a test which proves the crash occurs if the fix is not in place.

[coveralls]

coverage: 100.0%. remained the same
when pulling 1ecdfd4 on matthewhegarty:issue-1716-export-crash
into eabba74 on django-import-export:main.