Fix forge.update() for nested properties

[annakristinkaufmann]

The current behaviour of forge.update() is as follows:

• Example resource with a nested property:

  resource = forge.from_json({
  "outer": {
      "inner": "test"
  }
  })
  forge.register(resource)

• Updating the inner property throws a resource should not be synchronized UpdatingError:

  resource.outer.inner = "updated"
  forge.update(resource)

• While the following works fine:

  resource.outer = resource.outer
  forge.update(resource)