Bug 419: Hop by Hop headers MUST NOT be forwarded (attempt 2)

Bug 419: Hop by Hop headers MUST NOT be forwarded (attempt 2)

This attempt builds on Henriks re-work of the client-request to
server-request cloning done since the last attempt was made at closing
this bug.

Adds all RFC 2616 listed Hop-by-hop headers to the clone selection test
as 'ignore' cases unless otherwise handled already.

The test for whether they exist in Connection: is moved to the default
case as an inline. Which reduces the code a fair bit and prevents the
side case where a specially handled header gets ignored because the
client explicitly added it to Connection: when it did not have to.

This method sets up a background default of not passing the hop-by-hop
headers while allowing any code which explicitly sets or copies the
headers across to operate as before without interference.