RFC 2045 doesn't permit non-ASCII bytes, but some systems send them
anyhow. With this change, we accept them. This does make it harder to
validate quotedprintable data, but on balance this seems like the best
approach given the existence of systems that generate invalid data.

Fixes #22597

Change-Id: I9f80f90a60b76ada2b5dea658b8dc8aace56cdbd
Reviewed-on: https://go-review.googlesource.com/121095Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>