UACs should add H-I if they implement it

[worley@…]

In section 4.1 is:

Normally, UACs are not expected to include a History-Info header in an initial request as it is more of a Proxy function; the main reason it is allowed is for B2BUAs who are performing proxy-like functions such as routing.

This is not a good idea; a UAC that wants History-Info should add the initial hi-entry. The reason is that the UAC does not know that the next proxy that will modify the request-URI implements H-I; if the UAC does not provide an hi-entry, the UAS won't see the original request-URI. Conversely, adding "History-Info: <[request-URI]>;index=1" is not much more work than adding "Supported: histinfo", which we already assume the UAC will do.