overwrite option for update works now

So, we added overwrite support in 3.7 with the addition of mquery, but
it wasn't actually working... woops. This fixes that.
made another more explicit test for overwrite
fixed tests & small logic error
Properly casting the query for overwrite updates

Before, was just skipping over the casting, should be taken care of now
now using _walkUpdatePath instead of query.cast
fixed for different semantics; can now fully overwrite
fixed merge conflict
fixed conflict