[4.2] Database : ensure equivalent dates are stored

So that inputing a date with a different timezone than the application timezone, will not store the same date (as the timezone is not store), but at least an equivalent date.
Fix https://github.com/laravel/framework/pull/5882
As mentioned in the other ticket, it might be wise here to ensure the default timezone is different from the test date's timezone, or the test is useless. Perhaps force it to UTC. Or are test cases always in UTC, no matter what?