Bug 5844: Avoid strings cut on unhighlighing

Bug 5844: Avoid strings cut on unhighlighing

If a user is searching for several words and that one is contained in
another ( "mag ma" ), some characters are cut (removed from the DOM!).

To reproduce:
1/ search for "mag ma m"
2/ The result page contains "Magazine"
3/ "Mag" is highlighted
4/ Click on the "Unhighlight" link
5/ The "Magazine" words become "azine", "Mag" has been cut.
There is no way to show these characters again.

Test plan:
1/ Confirm the bad behavior without this patch
2/ Apply this patch
3/ search for "mag ma m"
4/ The result page contains "Magazine"
5/ "Mag" is highlighted
6/ Click on the "Unhighlight" link
7/ strings are unhighlighted and still in the DOM :)

Note that the Highlight link won't work very well (only "a" will be
highlighted, but considered as minor since this did not work before the
patch neither).

Followed test plan. Strings are no longer cut when unhiglighted.
Signed-off-by: Marc Veron <veron@veron.ch>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
(cherry picked from commit 2e4b574074ad8f66908cd22b466ba7a991696686)
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>