Fix 'split' method when using String not RegExp

[prmtl]

If regular expression for splitting is provided as String instead of RegExp, it breaks split method since String doesn't have desired properties.

Fixes #138

[thisfred]

Confirmed this fixes #152, +1 from me

[thisfred]

I do however see a similar thing happening with hints still (hit f and then type one of the hints:

resource://dactyl/styles.jsm: 248: TypeError: can't assign to properties of (new String("hint-positions")): not an object

There is an open issue for this already: #154

[pink-mist]

:+1: This fixes #153 for me (which seems to be similar to #152, which I didn't test)

[dkearns]

Thanks.