core: on context prune-for-scomp, leave an empty list for request headers. Normalize user-agent lookup.

(cherry picked from commit f4bb10c4dd9fb797a2beab2e44b1850cfa89e0a5)
core: fix args for transport ack.

(cherry picked from commit 2965f8c40573cd578bd191f73f10072d96662103)