bug#9159: 24.0.50; `undo' is not as good as it should be wrt property changes

[Drew Adams]

emacs -Q
 
In *scratch*, do this with cursor on the character `T' (for example):
 
M-: (put-text-property (point) (1+ (point)) 'foo 42)
 
The buffer then appears modified.
With point in the same place, do `C-u C-x ='.
That shows that there is property `foo' there, with value 42.
 
C-M-_ to undo the last change.
With point in the same place, do `C-u C-x ='.
That shows that there is property `foo' there, with value nil.
And it shows the buffer as no longer modified.
 
IOW, the buffer modification is indicated as corresponding with setting
property `foo' to 42, and undo puts it back to nil.
 
While correct, it would be better for the property `foo' to simply be
removed, and thus not have `C-u C-x =' show a `foo' value at all.  Undo
is not really undoing things to put them back they way they were, as
witnessed by `C-u C-x ='.
 
NOTE: Sending from a different Emacs version because of the recent SMTP
mail bugs.  This is the version the report is for:
 
In GNU Emacs 24.0.50.1 (i386-mingw-nt5.1.2600)
 of 2011-07-18 on 3249CTO
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.5) --no-opt --cflags
-Ic:/build/include'