Fix for array to string error in setFromPost()

I am not sure if this belongs in the:
* How-the-Hell-Has-This-Not-Been-Hit-Before Department
* Life-on-the-Bleeding-Edge Department
* Only-Found-at-Six-AM-Bug Department

To produce this, you need to get a TRUE from `if ($this['ownerid'] != $values['ownerid'])`, which is easy to simulate if you set an `ownerid` to `NULL` in the database and then edit said record from the backend.

Totally normal behaviour... right?