Fix Routine.multi

```perl6    
    use Test;

    plan 1;

    my class Foo {
        multi method test(Str $) {}
        multi method test(Int $) {}
    }

    my $test = Foo.^method_table<test>;
    ok $test.multi, 'Making sure a multi method says it is one';
```

Without this patch, the preceding test doesn't work.

For some reason, `t/spec/S32-io/IO-Socket-Async.t` fails with this patch, though.