bug#21391: 24.5; `thing-at-point' should return a string

[Eli Zaretskii]

> Cc: tino.calancha@gmail.com, 21391@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Mon, 7 Nov 2016 21:51:23 +0200
> 
> >> It's pretty clearly a bug: most other "things" behave otherwise (list,
> >> sexp, symbol). I'd very much rather we didn't retain conceptual problems
> >> like that just for the sake of compatibility.
> >
> > Not sure which behavior you have in mind.  The removal of properties?
> > Or something else?
> 
> Returning something other than a string or nil from thing-at-point.

Didn't the function behave that way for quite some time?  If so, how
come it's suddenly a problem?