Add test for get + serialize behavior

[Bajix]

Assuming DefineMap.prototype.get() was intended to perform similarly to can.Map.prototype.attr(), then it should return a PJSO with the values of all enumerable props assigned. Instead, it is only returning all serializable props, essentially making this call signature identically to DefineMap.prototype.serialize().

This PR is just for the test to demonstrate that this behavior is broken or has otherwise changed in a breaking way.