dynamic array + copy ctor

[vit via Digitalmars-d-learn]

Hello,
Why is copy ctor in this example not called?

```d
import std.stdio;

struct Foo {
    int i;

    this(int i){
        this.i = i;
        writeln("init: ", i);
    }

    this(ref typeof(this) rhs){
        this.i = rhs.i;
        writeln("copy: ", i);
    }
    ~this() {
        writeln("~dtor:", i);
    }
}

void main(){
    Foo[] foos;

    foos ~= Foo(1);

    while(foos.capacity > foos.length)
        foos ~= Foo(0);

    foos ~= Foo(2);

    import core.memory;
    GC.collect();
}
```
result:
```
init: 1
init: 2
~dtor:1
~dtor:2
~dtor:1
```

First Foo is destructed 2x.