utils: When serializing a dict, don't have the for loop _unpack_ the key, have it get the key/value.

I'm not sure how this worked before, and I'm sorry if it caused any issues, but my PR with this previously seems to not be working the way it should and instead causes errors from unpacking keys longer than 2 characters.

Anyway, this should actually serialize the dictionaries into `{key: serialize_obj(value)}`.