Interpolation in propsets doesn't work

[xzyfer]

div {
  back#{ground}: {
    imag#{e}: url(foo.png);
    pos#{it}ion: 50%;
  }
}

Ruby Sass

div {
  background-image: url(foo.png);
  background-position: 50%; }

LibSass

div {
  back#{ground}-image: url(foo.png);
  back#{ground}-position: 50%; }

Spec sass/sass-spec#455