Bug 10688: Don't prefix index inappropriately

Bug 10688: Don't prefix index inappropriately

If you select an index in the search dropdown and then enter in a QP
query starting with the field, Koha will prepend the index you do not
want to use at the beginning of the search, resulting in a search that
probably does not match what you were hoping for.

To test:
1) Select an index in the search dropdown in the OPAC. Author is fine.
2) Enter a search term using manually entered indexes. For example:
    ti:cat in the hat
3) Note that the search fails.
4) Apply patch.
5) Repeat steps 1 and 2.
6) Note that the search succeeds.
7) Sign off.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 03f342aa67034a56876b9b45545c607b961587b7)
Signed-off-by: Fridolin SOMERS <fridolin.somers@biblibre.com>