Fix overwriting updated_at when $set is used. Fix ignoring dateFromat

[guram-vashakidze]

Hello,

1. Found unexpected behaviour when I'm adding updated_at into $set in Eloquent update. For example:

MyModel::query()
    ->where('foo', 'bar')
    ->update(['$set' => ['...' => '...', 'updated_at' => Carbon::now()->toAtomString()]]);

In this case current implementation is overwrite updated_at

2. The $dateFormat is ignored when created_at and updated_at are added automatically (when model is saving) and every time set UTCDateTime as format for value in DB. Added check - if dateFormat is set in model -> Date::now() is returned from freshTimestamp instead of UTCDateTime(Date::now())

[guram-vashakidze]

UPD: 2nd commit is not solution. My fault.