Return a "datetime" rather than a "date".

Due to Erlang's inability to distinguish a date() value from a now() value, this helps to make it clear that it's a date.  Fixes https://github.com/ChicagoBoss/ChicagoBoss/issues/489

I'm not 100% sure about this, as it'll likely break things.  On the other hand, it does fix the JSON issue and make dates into datetimes.

I suppose it might also create some 'round trip' problems, so maybe it needs more work.  Thoughts welcome.