acme, acme/autocert: use canonical http header keys

Change-Id: I6aebbd0c351c1dcc27504f5a6fe02bfe7ebe805b
Reviewed-on: https://go-review.googlesource.com/47361
Run-TryBot: Alex Vaghin <ddos@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Vaghin <ddos@google.com>
4 files changed