Document that key in Header.Get(key) is case insensitive in
http.Header, mail.Header, textproto.Header.

Fixes #18019

Change-Id: Iba7932491e02e555190b6fce053088b580a853ef
Reviewed-on: https://go-review.googlesource.com/33530Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>