Update Model.php function asDateTime: startofDay(00:00:00) if from Y-m-d timestamp (DateAccessor)

As the pull request https://github.com/laravel/framework/pull/2227 was closed but the startOfDay() was only inserted in the function fromDateTime, this pull request adds startOfDay() also to the accessor function (asDateTime), so you can compare two dates without the automatically added incorrect time of the current moment.