acme/autocert: enable HTTP/2 on listener

Enables HTTP/2 on any servers used with the autocert listener
by setting "h2" in NextProtos of the listener *tls.Config.
Also adds a warning to the listener documentation that it
enables HTTP/2.

Fixes golang/go#20572

Change-Id: If7c0f5722f0b1781789219fc4e84da3f19a89ab7
Reviewed-by: Brad Fitzpatrick <>
Run-TryBot: Brad Fitzpatrick <>
TryBot-Result: Gobot Gobot <>
1 file changed