Disable commenting out blank lines by "Toggle line commentation"

When default comment marker "~ " is used and "Strip trailing spaces and tabs on file saving" is enabled, commented blank lines will not be uncomment after file saving:
![before](https://f.cloud.github.com/assets/1297574/287614/ed71db52-925b-11e2-9bf1-f6cd8109fd69.gif)

**With this patch:**
![after](https://f.cloud.github.com/assets/1297574/287626/3c3e7dda-925c-11e2-83fa-f7fcd160b9d4.gif)