webadmin: Small logic fix when reassigning roles

Previous fix implicitly relied on the state of the sent entity too
much, now comparison is made with the entity as stored in the DB.

Change-Id: I99bceea9f35f2356897a21c6e6e7f2f051f3f37d
Bug-Url: https://bugzilla.redhat.com/1135785
Signed-off-by: Lior Vernia <lvernia@redhat.com>