[xhr] proxy-connection header

The XHR spec currently allows users to set the "Proxy-Connection" header 
using setRequestHeader method. I couldn't find a spec for it other than 
some discussions here:

http://www.hpl.hp.com/personal/ange/archives/http-wg-archive/index.html#1214

However it seems like it basically does the same thing for a 
client-proxy connection as the normal "Connection" header does for a 
client-server connection.

Mozilla seems to set the "Proxy-Connection" header rather than 
"Connection" when knowingly talking to a proxy. See

http://bonsai.mozilla.org/cvsblame.cgi?file=mozilla/netwerk/protocol/http/src/nsHttpHandler.cpp&rev=1.129#387
(warning, licensed code, talk to your lawyer ;) )

So I think we should disallow this header since we're disallowing 
"Connection" as it might otherwise confuse proxies.

Best Regards,
Jonas