Correctly humanize keystrokes using both shift and capital letters

Fixes https://github.com/atom/feedback/issues/27

Currently, keystrokes such as `cmd-shift-P` are humanized as `⌘⇧⇧P`, instead of as `⌘⇧P`:

![screen shot 2014-03-07 at 3 47 58 pm](https://f.cloud.github.com/assets/38924/2358097/84f02668-a607-11e3-937e-594eed93e4d2.png)

This pull request adds a test to detect the behavior, and fixes the issue by removing duplicate keys from a single humanized keystroke. 

Since keys in a keystroke should be unique, removing duplicates _should_ always be okay. Also, since keystrokes are normally short, the performance penalty of using `_.uniq` shouldn't be noticeable.