[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 think this would be clearer and more efficient as `$this->assertEquals($inputDate, $outputDate)`, which would skip the conversions to string.