Previous implementation of `Document._object_key` was *pretending* to work on
MongoEngine-level fields (e.g. using "pk" instead of "_id" and separating
nested field parts by "__" instead of "."), but then it was also attempting to
transform field names from the `shard_key` into DB-level fields.
This, expectedly, didn't really work well. Most of the test cases added in this
commit were failing prior to the code fixes.
aa76ccdd25