#62 Adding ISA proxy required headers, not assured to be in original User Agent request

[Erik Antelman]

We have a proxy that requires the second request to have a "User Agent" header. In at least the case of a python request originator this "User Agent" was not being placed on the CONNECT. I tried to use the -r to force a value to be present.
I found that there is no way to ADD a header due to forward.c:497 having the add==0.

My solution was to change the add==0 to add/mod semantics.
Yet a better solution is add if not present.