`toJSON()` with `validatePropertyValues: true` treats choices whose `value` is an object as invalid

[andrewtelnov]

toJSON function doesn't process correctly property values that has choices attribute where value is object. It added an error where there is no errors.
Here is the example:

Serializer.addProperty("survey", { 
name: "prop1", choices: [ 
   { value: { point: 1, color: "red"} }, 
   { value: { point: 1, color: "blue"} }, 
   { value: { point: 2, color: "green"} }, 
]});