mime/multipart: add Part.NextRawPart to avoid QP decoding
NextPart has automatic handling of quoted-printable encoding,
which is sometimes undesirable. NextRawPart adds a method
for reading a part while bypassing such automatic handling.
Fixes #29090
Change-Id: I6a042a4077c64091efa3f5dbecce0d9a34ac7065
Reviewed-on: https://go-review.googlesource.com/c/go/+/152877
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>