Allow using domain specific models in testNullRelativeUri

Currently, *testNullRelativeUri* depends on a hard-coded model for testing null relative URIs. As described in https://github.com/w3c/ldp-testsuite/pull/35, this test always fails when the server is domain-specific.

**Proposal**
Use the *postContent* model (Bug or custom) and verify if the intersection between the requestModel and the responseModel is not empty.
I think this is better, the only conflict we'd get if someone uses http://example.org as a subject, small chance.  Could always create some crazy like http://w3c.github.io/ldp-testsuite/fakesubject which would have even less of a chance to conflict.