Fixed Rollback after POST IntegrityError & added test

See issue https://github.com/jfinkels/flask-restless/issues/356
Well i guess the SQLa Documentation describes it the best: http://docs.sqlalchemy.org/en/latest/orm/session.html#sqlalchemy.orm.session.Session.is_active

From the top of my head:
It basically tells you if you have a proper connection that has not failed to finish a transation and is therefor not in a so called "partial rollback" state. I'll add a message to the assertion.

The check on POST is in the 2nd commit of this PR. This should be fine to merge at once?