Superfluous UTF-8 BOM in messages/az/yii.php.
Later it is inserted into the answer on network.
See screenshot.

| Q | A
| ---------------- | ---
| Yii version | 2.0.10
| PHP version | 5.6.29
| Operating system | Debian 8.6
I scanned the entire framework directory. Indeed, this file has a BOM.
grep -rlI $'\xEF\xBB\xBF'
messages/az/yii.php
Used this command for scanning with additionally ignoring of binary files.
Good nitpick, I'll send PR in the next few minutes.
added a check to travis.yml: 17c2725
Good idea to automate detecting BOM in Travis, 馃憤
Most helpful comment
added a check to travis.yml: 17c2725