http: free hdr->key and hdr->value

Fixes memleak

Change-Id: Ia384389a5ea45e5098102aa3c0e1d26329e635c3
Reviewed-on: http://review.couchbase.org/33107
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>