Strip units from zero value.

It is best practice to omit units from zero value in CSS. Your IDE may also warn you so (Sublime Text does). The changes in this branch return "0" without the units, so if you do something like:

@include rem-fallback(padding, 0, 1.2rem, 1rem, 0);

That should result in:
  padding: 0 1.2px 1px 0;
  padding: 0 1.2rem 1rem 0;

Not:
  padding: 0px 1.2px 1px 0px;
  padding: 0rem 1.2rem 1rem 0rem;

BTW, thanks for the mixin!