bug#12986: Pressing backspace during isearch exits case-sensitive and word modes

[Juri Linkov]

> Using 24.2, type:
> onto to
> Then do:
> C-a C-s to
> It highlights the first occurrence of "to" as the current match,
> and the second as the next match.
> Then do: M-s w
> Now, if you type a space, it continues in word mode as expected, but
> if instead you backspace over the o, it exits word mode. It shouldn't do that.
> This same problem applies to case-sensitivity mode (M-c) too.

<backspace> (isearch-delete-char) restores the previous search state.
If you want just to remove the last character from the search string,
you can type `C-M-w' (isearch-del-char).