v4 use letf for text/enriched bugfix.

[David Bremner]

This adds a comment to Tomi's previous version, and a test. I've
tested the test by commenting out Tomi's fix and running it under
emacs24 ("exploit" runs) and debian emacs 25.2 (which includes the
relevant fix. In the latter case, the test passes with the notmuch
code commented out, because emacs mime-rendering has been fixed. I
consider this reasonable, since it test success means this particular
exploit is blocked.