Аdded support for pixel values ​​for "size" mixin

There is some problem in this case:

$width: 350;
$height: 200;

.wrapper {
    @include size($width $height);
}

Sass compiles nothing and don't show  any errors, this looks a little bit strange. This fix can help with compiling mixin for every type of value(100, 100px, auto).