Bug 20110: Adding same user multiple times to same budget

Bug 20110: Adding same user multiple times to same budget

In javascript, using indexOf to test if a number exists in an array
of strings doesn't work, and makes it possible to add the same user
multiple times into the same budget fund.
Make borrowernumber into string and compare that instead.

Test plan:

1) Go to Home -> Administration -> Budgets -> Funds -> Add Fund
2) Add a user to the fund
3) Try to add the same user again to the same fund. No error.
4) Apply patch
5) Do 1, 2, and 3 again. You should now get an error message
   saying the user is already in the list.

Signed-off-by: Pasi Kallinen <pasi.kallinen@joensuu.fi>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>