Use ActiveRecord#persisted? instead of ActiveRecord#new_record?

When record was destroyed, it will return false when new_record? called :
```
u = User.first
>> u.new_record?
=> false
>> u.persisted?
=> true

>> u.destroy

>> u.new_record?
=> false
>> u.persisted?
=> false
```
I think persisted? is more appropriate here.
Fix specs
Merge pull request #600 from tubaxenor/patch-1

Use ActiveRecord#persisted? instead of ActiveRecord#new_record?