Fix last line commenting

Fix behavior of `Edit -> Format -> Comment Line(s)`:
If last line contains only one character, it's detected as empty and is not commented out.

**Before patch:**
![before](https://f.cloud.github.com/assets/1297574/282996/7c4fec0c-91a2-11e2-87cd-c0a8ae3aa0a5.gif)

**After patch:**
![after](https://f.cloud.github.com/assets/1297574/283011/d111762a-91a2-11e2-9c10-8fe8522c5ce2.gif)