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

[Kelly Dean]

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.