net/mail: do not parse RFC 2047 tokens in quoted strings

RFC 2047 tokens like =?utf-8?B?whatever?= can only appear
unquoted, but this code was trying to decode them even when
they came out of quoted strings. Quoted strings must be left alone.

Fixes #11294.

Change-Id: I41b371f5b1611f1e56d93623888413d07d4ec878
Reviewed-on: https://go-review.googlesource.com/17381


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>