Only include box-sizing if a value is given

[hashworks]

Without this I had a small bug using susy and its span mixin:

@import 'susy';
@import 'compass/css3/box-sizing';

@media (max-width: 720px) {
    .foobar {
        @import span(first to 12);
    }
}

@media (max-width: 720px) {
    .foobar {
        -webkit-box-sizing: ;
        -moz-box-sizing: ;
        box-sizing: ;
        width: 100%;
        float: left;
        margin-left: 0;
    }
}

Note that this only happens when used within a @media query, not sure how this is related.

[xzyfer]

This was due to a bug in node-sass which is now fixed. Tests add in #90.