net/textproto: initialize commonHeader in canonicalMIMEHeaderKey

net/textproto: initialize commonHeader in canonicalMIMEHeaderKey

Call initCommonHeader in canonicalMIMEHeaderKey to ensure that
commonHeader is initialized before use. Remove all other calls to
initCommonHeader, since commonHeader is only used in
canonicalMIMEHeaderKey.

This prevents a race condition: read of commonHeader before
commonHeader has been initialized.

Add regression test that triggers the race condition which can be
detected by the race detector.

Fixes #46363

Change-Id: I00c8c52c6f4c78c0305978c876142c1b388174af
Reviewed-on: https://go-review.googlesource.com/c/go/+/397575
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>