Need to first make it a Set before making it a Mix

E.g. turning a Hash to a Mix assumes numeric values for the keys in the
hash.  We shouldn't do this.  Spotted by Norman Gaywood++ on perl6-users.

src/core/set_difference.pm

@@ -141,7 +141,7 @@ multi sub infix:<(-)>(**@p) {
             nqp::create(Rakudo::Internals::IterationSet)
           ),
           nqp::unless(                           # something else, Mix it!
-            $p.Mix.RAW-HASH,
+            $p.Set.Mix.RAW-HASH,
             nqp::create(Rakudo::Internals::IterationSet)
           )
         )),