bug#21471: 25.0.50; bug report with text from Info has spurious escape chars

[Paul Eggert]

This bug is not a regression, as I get similar behavior with Emacs 24.5 on 
Ubuntu 15.04 (email client Thunderbird 38.2.0) as follows:

emacs -Q
C-h i m elisp RET
C-x h M-w
M-x report-emacs-bug RET
test subject RET
C-y
C-c C-c yes RET
mail client RET

The mail that's sent includes strings like this:

Copyright =C2=A9 1990=E2=80=931996, 1998=E2=80=932014 Free Software Foun=
dation, Inc.

even though it's marked "Content-type: text/plain; charset=utf-8; 
format=flowed".  This problem occurs with all non-ASCII characters, not merely 
with curved quotes.