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

[Eli Zaretskii]

> Date: Mon, 14 Nov 2016 09:33:00 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> Cc: dgutov@yandex.ru, tino.calancha@gmail.com, 21391@debbugs.gnu.org
> 
> All this just means that what you call the "caller", function
> `thing-at-point',

No, the caller is whoever calls thing-at-point the function.  Only
that caller knows whether the text properties on any text the function
returns are wanted or not, which is why the caller can use the
NO-PROPERTIES argument to express the intent.