bug#31073: 27.0.50; query-replace undo might change the word case

[Juri Linkov]

> The undo feature shouldn't change the case; it must return
> to the original words.
>
> emacs -Q
> <
> M-% t RET FOO RET SPC SPC SPC SPC U
> ;; The first line shows upcase 'T' at several positions, for instance
> ;; it shows 'TexT', originally shown as 'text'.

Good catch!  This means we should have more test coverage.  Thanks.