We've gone from Prefer: representation=secure to Prefer: https to Prefer: tls. That's pretty good, 11 characters, 13 bytes on the wire, but should we go further?

An even shorter option which I'm stealing from #212 is Upgrade: 1, 10 characters. Or we could do Pref: tls, 8 chacters. Or go all the way to TLS: 1 or HS: 1 for "hypertext secure", 5 characters, 7 bytes on the wire?

There are some aesthetic / clarity arguments for stopping at some point along this line, and I feel I lack data for where to put the pin.