v2 - implement tests for routing issues

[baywet]

In #2041 and #2042 I've fixed issues where due to the layering of the serialization code, a V2 serialization method ended up calling a v3 serialization method when it shouldn't have.

We should implement use tests controlling for that with spies where we effectively:

• spy all v3 methods for objects that have a v2 serialization method
• populate a large DOM, trying to cover all API surface
• run v2 serialization
• control that no v3/3.1 method has been called

We should probably implement the other way around (v3 doesn't call v2) and deserialization tests as well