add htparse keepalive tests

Change-Id: I0d04ca77f05678fa5f7e04a7b1089a695bf4dfda
Reviewed-on: http://review.couchbase.org/43386
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Subhashni Balakrishnan <b.subhashni@gmail.com>