[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
I asked the first ticket to be sent to 4.2 because it was a failing test to demonstrate a bug.

This pull request is now a change in behaviour.