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

[Drew Adams]

> This is already fixed in the current sources.  We now only remove
> the text properties if the object supports that.  Why isn't that enough?

My understanding was, as I said, that Emacs Dev wanted `t-a-p' to
always return a string (or nil), and that `form-at-point' is to be
used to return something else.  The advantage of that, I suppose,
was considered to be that callers could count on the return type,
without regard to the THING arg.

I don't have emacs-devel or bug-report messages to hand, to
support my understanding, but I thought that was the point of view
taken by those deciding, so far, wrt thingatpt.el.  I know that I
made an effort to adjust my code, to accommodate that approach.