Respect tzinfo in the provided datetime

[tschaub]

It looks like the fix for #56 got closer to being able to represent dates as RFC-3339 formatted strings. However, it looks like any tzinfo in the provided datetime is ignored. So the current utils.datetime_to_str() function works for timezone naive datetimes, but not for timezone aware datetimes.

This branch proposes to treat timezone naive datetimes as UTC datetimes (in line with the current assumptions) but respects any tzinfo in the provided datetime.

[lossyrob]

Thanks for the PR @tschaub !

All the examples in the stac spec use the Z representation for UTC, as opposed to +00:00. I'd like to keep that for keeping in line with those examples - perhaps if the tzinfo is None or matches UTC, use the Z, else use the iso format?

[tschaub]

Thanks for the nice library, @lossyrob.

I updated the branch to prefer the "Z" suffix for zero-offset timestamps.

[lossyrob]

Looks good!