fixed datetimes not being converted in models

[josephmancuso]

Closes #513
@girardinsamuel This should work right?

[girardinsamuel]

Maybe... but I am not sure where this method is called. Maybe it would be ideal to write a unit test to cover those use cases:

import pendulum
now = pendulum.now()
# use case 1
User.create({"name": "John", "verified_at": now})
# use case 2
user = User.find(1)
user.update({"verified_at": now})
# use case 3
user = User.find(1)
user.verified_at = now
user.save()