[5.6] syncOriginal on refresh

[patrickomeara]

This is a new PR from #21889 with target version 5.6, instead of 5.5

Currently, after refreshing a model it can be dirty even when the attributes match those in the database.

$post = Post::create(['title' => 'pat']);

Post::find($post->id)->update(['title' => 'patrick']);

$post->refresh();

// current result
$post->getOriginal('title'); // pat
$post->getDirty(); // ['title' => 'patrick']

// with proposed fix 
$post->getOriginal('title'); // patrick
$post->getDirty(); // []

Syncing with the original after refresh prevents this.