Add spec for LibSass issue 2681

[skip libsass]

Author: xzyfer

spec/libsass-closed-issues/issue-2681/input.scss

@@ -0,0 +1,12 @@
+%button-styles {
+  color: red;
+
+  &:focus {
+    color: blue;
+  }
+}
+
+[type="button"] {
+  @extend %button-styles;
+}
+

spec/libsass-closed-issues/issue-2681/output.css

@@ -0,0 +1,6 @@
+[type="button"] {
+  color: red;
+}
+[type="button"]:focus {
+  color: blue;
+}