fix bug 881342 - Don't clear field after suggestion selection

https://bugzilla.mozilla.org/show_bug.cgi?id=881342

To test:

1.  Go to the Rev Dashboard
2.  Type in the first three letters of your username
3.  When the autosuggest pops up, keyboard down to your user
4.  Press [ENTER]
5.  See the field populate with your username.
6.  Smile
7.  Press "Merge button"