The existing implementation correctly supported RFC 5322, this
change adds support for UTF-8 while parsing as specified by
RFC 6532. The serialization code is unchanged, so emails created
by go remain compatible with very legacy systems.

Fixes #14260

Change-Id: Ib57e510f5834d273605e1892679f2df19ea931b1
Reviewed-on: https://go-review.googlesource.com/19687
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexandre Cesaro <alexandre.cesaro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>