Get rid of DateTime in StripeTesting

[begedin]

Problem

stripity_stripe has stopped using DateTime and instead now relies on integer unix timestamps only. Our StripeTesting module, located in various files under lib/code_corps/stripe_testing should also stop using DateTime.

This will also allow us to remove the method signature for stringify_keys from MapUtils.

Subtasks

• Go through lib/code_coprs/stripe_testing and replace all instances of DateTime.from_unix(integer_value) to just plain integer_value
• Remove defp stringify_keys(%DateTime{} = val), do: val from lib/code_corps/map_utils
• Make sure tests still pass

[afsharalex]

Hi,

Should I replace DateTime as {:ok, date} = {:ok, integer_value}, or remove :ok`
Thanks.