shims/validatejs.js - mandatory can.Map attributes with undefined value are not validated

[fantanamihai]

Undefined attributes within a can.Map, that have a corresponding define.validate object, are not validated with validatejs.
The cause is that the validate method in shims/validatejs.js receives as first parameter can.Map.prototype.serialize(), which by default doesn't serialize undefined values. Then it filters the attributes with Object.keys(), thus fails to validate attributes with corresponding define.validate object, but no value e.g.
can.Map.extend('Test',{ define: { a: { validate: { required: true } }} }

new Test().validate() returns true

A workaround is all attributes to have a default value of 'null'. But is it desirable?