Error when changing object field to string

Currently, there is an issue when diffing and patching an object where the types of a field changes from an object to a string.

That is, I would expect the following to work:
```
var a = {hello: {nested: 2}};
var b = {hello: 3};
var changeset = diff(a, b);
var b_ = diff.apply(changeset, a);
expect(b).to.deep.equals(b_);
```
However, `b_` is set to `{hello: {}}` following the above operations.