#1158 different arrData between create and save - isAttributeSet get not the same returns (state: open)

Checklist before I submit this issue report

I confirm that:

• I have tested this with the latest version available
• I have read documentation @ http://metamodels.readthedocs.org/en/ or http://metamodels.readthedocs.org/de/
• I have checked the Contao community forums for references https://community.contao.org/
• I have checked existing issues for duplicates and found none @ https://github.com/MetaModels/core/issues?q=is%3Aissue

My environment is:

(Please fill in the actual values from your environment)

Steps to reproduce

1. create attribute Alias with allwaysSave
2. create input mask with readonly
3. create and save items

look at https://github.com/contao-community-alliance/dc-general/issues/350

at the "first save" (create) we doesn´t create the alias - we create the alias with the "second save" (update)

the MetaModels.php call the method modelSaved if is isAttributeSet true https://github.com/MetaModels/core/blob/08b502234efe150003450cf01b5611b00f9ebd57/src/MetaModels/MetaModel.php#L1035 but at Item.php https://github.com/MetaModels/core/blob/08b502234efe150003450cf01b5611b00f9ebd57/src/MetaModels/Item.php#L245 we have different keys at array $this->arrData between create e.g.

Array (     [name] => a8     [vorname] => b8     [published] => 1     [tstamp] => 1496848172     [id] => 10 )

and update e.g.

Array (     [id] => 10     [pid] => 0     [sorting] => 0     [tstamp] => 1496848200     [name] => a8     [vorname] => b8     [email] =>      [published] => 1     [abteilung] =>      [alias] =>  )