Fix KA header not checking strategy (#7483)

Fixes handle_request_keep_alive_headers directly checking the old
parentage for parent_is_proxy, instead of the abstract func which
checks the strategy if it exists.

This would cause the Connection header to get added instead of the
correct Proxy-Connection when Strategies are used.

(cherry picked from commit e24c79a4a530dc9b12880503e8c047e5610c3224)
1 file changed