Support for height: auto;

[Baedda]

I have a long option that breaks into a second line, so I cannot set a fixed $input_height. But when I try to set $input_height to auto I get the following warning on the console:

Warning: math.div() will only support number arguments in a future release.
Use list.slash() instead for a slash separator.

Its because of the following code
transform: scale(.75) translateY(- math.div(-$input-height, 2));
out of https://github.com/bluzky/nice-select2/blob/master/src/scss/nice-select2.scss#L126

[Tsjippy]

I cannot test, please create a pr for this