Avoided COMP_WORDBREAKS modification

Modifying `COMP_WORDBREAKS` breaks ssh/scp completions (for example, `scp host:<Tab>` shrinks to `scp /home/dir/` without the host part).  Found a [good answer from stackoverflow](http://stackoverflow.com/a/12495480/390044) for this problem, and simply applied the knowledge here.  Please merge.

~Jaeho