net/mail: added support to trailing CFWS in date

RFC 5322 date format allows CFWS after the timezone.
If CFWS is valid, it is discarded and parsing is done as before
using time.Parse().
Existing test is extended with limit cases and invalid strings.

Fixes #22661

Change-Id: I54b96d7bc384b751962a76690e7e4786217a7941
Reviewed-on: https://go-review.googlesource.com/c/go/+/117596


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>