Observable.get() erases typing of private fields

[simontreny]

Consider the following pieces of code:

class Foo {
  private bar: string;

  constructor(bar: string) {
    this.bar = bar;
  }
}

const obs = observable(new Foo("bar"));

const val: Foo = obs.get();

This raises the following typescript error: Property 'bar' is missing in type '{}' but required in type 'Foo'

Expected behavior: Observable<T>.get() should always return T on a non computed observable.

Tested version: 3.0.0-beta.16