Don't invoke HTTP callbacks after cancellation

Otherwise, user code might assume a previously-freed resource is still
valid

Change-Id: I7cfdea8b9c9544d24622bb92e419096e97aa8a37
Reviewed-on: http://review.couchbase.org/25520
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>