Use the multiple property of rule operator instead of filter

[matthewma7]

I have similar needs as this #474

But as it is pointed by the author, changing the select element multiple attributes won't work (without special handling). #474 (comment).

But we have the multiple property at operator level.
Could we use rule.operator.multiple instead of filter.multiple here?
https://github.com/mistic100/jQuery-QueryBuilder/blob/dev/src/data.js#L378

[mistic100]

This is need a bigger investigation of the possible breaking changes this induces.
But it makes sense :-)