Add failing unit test related to timezones

When a date attribute is assigned a Carbon object with a timezone
other than the application's configured timezone, it is not taken into
account and the time is changed by whatever the offset is.

This is a failing test for the issue described in
https://github.com/laravel/laravel/pull/3044

This is a patch against 4.2 rather than master as requested in
https://github.com/laravel/framework/pull/5879