Replace references to the obsoleted RFC 2616 with references to RFC
7230 through 7235, to avoid unnecessary confusion.
Obvious inconsistencies are marked with todo comments.

Updates #21974

Change-Id: I8fb4fcdd1333fc5193b93a2f09598f18c45e7a00
Reviewed-on: https://go-review.googlesource.com/94095Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>