http: prevent segfault during curl handle reuse

If a curl handle is configured with special options, they may reference
information that is freed after the request is complete which can cause
a segfault if the curl handle is reused for a different type of request.

This patch resets these options to a safe state when a transfer slot is
assigned to a new request.

Signed-off-by: Nick Hengeveld <nickh@reactrix.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>