Currently the @optional tag overrides class properties defined with typescript-style default values:
```typescript
@Injectable()
class Ninja {
    @Inject(Weapon) @optional() weapon: Weapon = new Katana();

    ....
}
```
this currently sets the weapon property to `undefined` if no Weapon is present in the container. My proposed change allows does not override the default and results in the weapon property being set to new Katana().

(Default values in the constructor already work, but I would prefer not to add boilerplate constructors if this solution is possible.)