feat(collapse): Bootstrap3 compatibility/bugfixes

Same issues as #934:
- Make code more concise (I could use help with this, as I've said)

---

Short of this, tests are passing, and from what I've seen, we're getting
good behaviour from all browsers. So this is good.

See #934 for other work on this same problem, I'd like to get this merged
in, for the benefit of my own apps and everyone elses :]