This lets quotedprintable handle some inputs found in the wild,
most notably generated by "Microsoft CDO for Exchange 2000",
and it also matches how Python's quopri package handles these inputs.

Fixes #13219.

Change-Id: I69d400659d01b6ea0f707b7053d61803a85b4799
Reviewed-on: https://go-review.googlesource.com/32174Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>