Bug 27348: Fix test on INDEXER_PARAMS in koha-indexer

Bug 27348: Fix test on INDEXER_PARAMS in koha-indexer

koha-indexer doesn't test INDEXER_PARAMS correctly which causes errors
to display when stopping/starting the daemon.

This patch fixes the test so that the variable is tested as a string,
so that no errors are created and the params are passed correctly.

Test plan:
0. Apply patch
1. vi /etc/default/koha-common
2. Add the following to the bottom of the file:
INDEXER_PARAMS="-daemon -sleep 6"
3. cp debian/scripts/koha-indexer /usr/sbin/koha-indexer
4. koha-indexer --stop kohadev
5. Note no errors
6. koha-indexer --start kohadev
7. Note no errors
8. ps -efww | grep "indexer"
9. Note that rebuild_zebra.pl has the arguments "-daemon -sleep 6"

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>