UPGRADE: 'HTTPS' header causing compatibility issues.

Chrome 44 sends an `HTTPS: 1` header, as specced at
https://w3c.github.io/webappsec/specs/upgrade/#preference. It looks like
this is causing issues with some folks' servers. These are the bugs I've
seen reported so far:

* https://crbug.com/501095
* https://crbug.com/501842
* https://crbug.com/504357

My vague guess is that some configurations set internal variables based on
header names (e.g. `HTTPS: 1` => `$HTTPS == 1`), which is confusing the
poor programs.

With this in mind, I think it might be advisable to change the header name,
which means diving back into the bikeshed of
https://github.com/w3c/webappsec/issues/216. Think we can agree on a name
this week?

If we can't, I'll run with `TLS: 1` by royal fiat. :)

--
Mike West <mkwst@google.com>, @mikewest

Google Germany GmbH, Dienerstrasse 12, 80331 München,
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(Sorry; I'm legally required to add this exciting detail to emails. Bleh.)