Fix findSiblings and the target kept its prototype

[sapryniukt]

We ran into the issue of losing prototype and incorrect invocation of setDeep when tried to change the EmberData property in the changeset.

My changeset looked something like this:

data: {
    "name": "Jim Bob",
    "address": CountryModel {
        "country": "US",
        "address1": "",
    }
}

but after execution:

changeset.set("address.address1", "some");

We got:

changes: {
    "name": "Jim Bob",
    "address": Object {
        "address1": "some",
    }
}

Looks like findSiblings works well with POJO, but doesn't consider Class object.
I proppose a simple solution to this by using

Object.setPrototypeOf(obj, prototype)

or

Object.create(prototype)

Also the tests that check and reproduce the issue have been added.