Fix validation of required properties not defined in the properties section of a schema

Previously to this release a schema like the following won't validate the property bar:

{
  "type": "object",
  "properties": {
    "foo": {
      "type": "string"
  },
  "required": [
    "foo",
    "bar"
  ]
}

Now the property will be validated and added to the interface of the generated class. Generated interface:

getFoo(): string;
setFoo(string $value): self;
getBar(): mixed;
setBar(mixed $value): self;