Properties validation weren't implemented

[xeronm]

The following schema validation succeeded, even though the prop1 reference is broken.

openapi: 3.0.0
info:
  version: '1.0.0'
  title: 'Some Schema'
paths: {}
components:
  schemas:    
    SomeDataType:
      type: object
      properties:
        id:
          type: integer
        prop1:
          $ref: '<some-broken-uri>'
        prop2:
          type: string