Two ModelRefs with key == null should be equal

[krico]

Fixes #19

[krico]

To provide a little explanation why I think this is a problem, suppose you have the following Model

@Model
class Foo {
  private Key id;
  private String name;
  private ModelRef<Bar> barRef = new ModelRef<>();
  public boolean equals(Object o) {
    Foo f = (Foo) o;
    return id.equals(f.id) && name.equals(f.name) && barRef.equals(f.barRef);
  }
}

Foo foo = new Foo(id, name);
foo.equals(foo); //yields false