Both `http-kit` and `ring.util.codec/form-encode` use an invalid (according
to some, Stripe included) way of form-encoding arrays. This ns is copy of the
aforementioned Ring implementation with a minor modification -- see line(s)
marked as such below.

see also:
http://stackoverflow.com/questions/6243051/how-to-pass-an-array-within-a-query-string