Nested declarations are not spec compliant

[stof]

This is about things like that:

a {
  b: {
    c: d;
    e: f;
  }
}

which should produce a CSS like that:

a {
  b-c: d;
  b-e: f;
}

[stof]

The basic feature (described in the issue) works (at least for a single level of nesting). However, it is broken when the inner declarations are generated with a mixin.