CCBC-537: cancel_http_request should call request_finish

This fixes some bugs where the request wouldn't be decremented.

Change-Id: If27026fde886983a1820612e2ffd8ba75ad7e8ae
Reviewed-on: http://review.couchbase.org/42614
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>