Fix for using {{render}} with same template multiple times

Yehuda added support for using `{{render}}` with same template multiple times with different models in 47dfe5ed39bade9f261b9049e7e5636bf056d718. But when using it, Ember threw an assertion error, `"This view is already rendered"`, for everything but the first `{{render}}` call.

This should be fixed now, and I added a test for it and adjusted the assertion error message.