Verify that revisions with identical content have the same rev id.

I added a test to verify the behavior on ios w.r.t revision id's being equal for revisions w/ identical content.  (this test would currently fail on cblite-java).

If there isn't already a test for this, I think it makes sense to add this.

Related to: https://github.com/couchbase/couchbase-lite-java-core/issues/164