mime/multipart: quoted-printable check is not quote enough

[jmhodges]

I've found emails that are Content-Type: multipart/alternative, but instead of putting
the Content-Transfer-Encoding: quoted-printable in each relevant part's MIME header
section, it puts it in the emails top-level MIME header section.

This makes the check in mime/multipart's newPart function[1] miss the fact that the
reader needs to be wrapped in quotedPrintableReader.

Either we could inherit the Content-Transfer-Encoding from the top-level header when
creating a new Part, or we can expose the quotedPrintableReader struct for users to fix
up.

[1] http://golang.org/src/pkg/mime/multipart/multipart.go#L98

[jmhodges]

Comment 1:

Shoot. I am mistaken. These weren't coming in as multipart. They were just plaintext
emails with Content-Transfer-Encoding: quoted-printable. There's just no public
QuotedPrintableReader to reuse. This can be closed in favor of something like #4943.

[jmhodges]

Comment 2:

As a mea culpa to people who come across this later, I've taken the QPReader from inside
mime/multipart and pushed it to as github.com/jmhodges/emailenc/quotedprintable. I might
end up adding a q decoder to it as well.

[ianlancetaylor]

Comment 3:

Status changed to Retracted.