fix val method

[SimonTod]

The val function does not work with integer values.

How to reproduce :

• checkout main or dev branch.
• open tests/bootstrap5.html
• open browser console
• run following js in console :

$('#number').selectpicker('val', 1); // Does not work
$('#number').selectpicker('val', "1"); // Works

for a given

<option value="1">Option 1</option>

both 1 (integer) and "1" (string) values should work.