Adds Keep-Alive to the list of ignored headers in encodeHeaders
as required in the HTTP/2 spec (section 8.1.2.2) and adds a test
to check this.

Fixes golang/go#15085

Change-Id: Ie4624680c5de1f13eb94fa58a2d5d67a02634df3
Reviewed-on: https://go-review.googlesource.com/21482Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>