Mongo data_store doesn't handle timestamps properly

[mrerrormessage]

In using the mongo data store, I've found the issue with timestamps. The mongo driver returns ruby time objects instead of strings, which causes issues in repository._deserialize. I'm not sure what the preferred solution would be here. I can see a few possibilities:

• Have repository check if the time passed back attributes[:create_at].is_a? Time (this is special-casing the repository for a driver)
• Have data_stores handle time serialization/deserialization (this seems rather workable, but would take longer and move more code out of repository)
• Have the mongo data_store put its timestamps in string format (hacky)

I'd be happy to put in work on whichever of these approaches seems best for the project!

[pgr0ss]

I think I prefer #2: have the repository assume that created_at/updated_at always come back from the data store as Time objects.