this might be a quibble, but it was a bit hard to understand that this function was checking for keys *besides* the reserved keys. it might be simpler to write `has_mongoose_reserved_keys` and then stick a `not` in front of the invocation.

also, you might want to leave a comment explaining what this check is about/why it's necessary. it seems like it might be non-obvious (or maybe it would be clearer to say "it seems like it might not be obvious" :stuck_out_tongue_winking_eye:)