bug#12987: Mode switching during isearch isn't user friendly

[Kelly Dean]

This is a UI improvement suggestion, not a bug report.
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
It continues to highlight the first occurrence of "to", even though that 
occurrence no longer matches. It should remove the highlight immediately. 
Pressing C-s again will remove the highlight, which is not user-friendly; 
pressing C-s should never cause highlights to be removed.
This same problem applies to case-sensitivity mode (M-c) too.