Regression with @extend breaks foundation

[xzyfer]

#2052 introduced a regression (d83f879) which is breaking foundation.

.media-object-section:last-child:not(:nth-child(2)) {
  color: blue;
}

%orbit-control {
  color: red;
}

.orbit-previous {
  @extend %orbit-control;
}

LibSass 3.4 RC

.media-object-section:last-child {
  color: blue; }

.orbit-previous {
  color: red; }

LibSass 3.3.6

.media-object-section:last-child:not(:nth-child(2)) {
  color: blue; }

.orbit-previous {
  color: red; }

Spec added sass/sass-spec#939

[xzyfer]

/cc @mgreter