Schema validation: unions

[ghostdogpr]

In Validator.validateSchema, verify the rules for union types

1. A Union type must include one or more unique member types.
2. The member types of a Union type must all be Object base types;
   Scalar, Interface and Union types must not be member types of a Union.
   Similarly, wrapping types must not be member types of a Union.

[palanga]

Can I take this ?