Form Editing, multi-select: optionally return array of selected values instead of comma-separated string

For server-side handling comparibility with regular HTML forms,
in Form Editing mode, for multi-select field, allow ``arrayValue`` editoption, which - if set to True - will return an array of selected options, instead of comma-separated values list.

example: we have select field and selected options with values of 10 and 11. 
Normally, jqGrid Form Edit would send to the server the value of ``"10,11"`` - which is comma-separated list of selected values. However, if we want to retrieve an array of selected values, like ``['10','11']`` - there is a new option ``arrayValue: true``.
```
{
...
edittype: 'select',
editoptions: { arrayValue: true },
}
```