std.concurrency : sending immutable classes

[Dicebot]

Updated case after some experiments:
```
import std.concurrency;
import std.stdio;

class A
{
     override string toString() const // this was an issue, it 
must be exact override
     {
         return "aaa";
     }
}

void thread()
{
     for (;;)
     {
         receive(
/*            (immutable(A) x) {
                 writeln("what");
             }, */
             (Variant x) {
                 writeln(x.type);
                 writeln(x);
             }
        );
     }
}

void main()
{
     auto tid1 = spawn(&thread);
     auto val = new immutable A();
     tid1.send(val);
     for (;;) {}
}
```

This prints expected:
```
immutable(test.A)
aaa
```

However, uncommenting `immutable(A)` receiver makes it silent 
again.