Fixed ProducedType.minus() to deal with supertypes of simple types as…

… it did for unions

This means that null.minus(Nothing) is now Bottom, for example, where previously it would
only work in the case of union types: (null|Integer).minus(Nothing) -> Integer

src/com/redhat/ceylon/compiler/typechecker/model/ProducedType.java

@@ -303,7 +303,7 @@ else if (p.isContravariant()) {
      * given type.
      */
     public ProducedType minus(ClassOrInterface ci) {
-        if (getDeclaration().equals(ci)) {
+        if (getDeclaration().equals(ci) || getSupertype(ci) != null) {
             return getDeclaration().getUnit().getBottomDeclaration().getType();
         }
         else if (getDeclaration() instanceof UnionType) {