Collapse: count `.collapsing` elements as actives; closes #13447

Created by: hnrch02

Fixes #13447 (closed)

Before the fix: http://jsbin.com/sayeg/1 After the fix: http://jsbin.com/sayeg/2 (Best reproducible when opening collapse 3 and then collapse 2.)

Tried writing a unit test but due to us disabling transitions there, it wasn't possible.

/cc @fat @cvrebert