http2: add a few other common headers to the shared headermap cache
http2: add a few other common headers to the shared headermap cache

This change adds additional common CORS headers and two de-facto
standard and common X- headers to the shared headermap cache to cut down
on allocations from lower-casing/canonicalization.

Change-Id: I61121925b0b28414ed6ce07190155662b0444f93
Reviewed-on: https://go-review.googlesource.com/c/net/+/442176
Auto-Submit: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

golang/net