ES6: Added string-spreading to the spread tests

A recent [spec change](http://wiki.ecmascript.org/doku.php?id=harmony:specification_drafts#july_18_2014_draft_rev_26) permits strings to be spread, as Firefox currently allows. I added that test to both spread tests, as well as checks that non-iterables throw a TypeError when spread.

**Note**: I accidentally based this branch off #164, so you ought to merge that before considering this.