Add todo spec test for libsass issue 131

sass/libsass#131

Author: mgreter

spec/libsass-todo-issues/issue_131/expected_output.css

@@ -0,0 +1,2 @@
+div {
+  content: "foo bar"; }

spec/libsass-todo-issues/issue_131/input.scss

@@ -0,0 +1,5 @@
+$foo: bar;
+
+div {
+    content: "foo #{$foo}"
+}