Add failing test for #9562

Rails 4.0.0 fails when trying to encode an ActiveSupport::TimeWithZone
that wraps a DateTime instance. This is fixed on master so add a test
to prevent regression.
Keep sub-second resolution when wrapping a DateTime value

Add `DateTime#usec` and `DateTime#nsec` so that `ActiveSupport::TimeWithZone`
keeps sub-second resolution when wrapping a `DateTime` value.

Fixes #10855
(cherry picked from commit 17f5d8e062909f1fcae25351834d8e89967b645e)

Conflicts:
	activesupport/CHANGELOG.md