read required elements in composed models.

[HugoMario]

Required properties are not properly read from a composed schema, for example, in this example:

definitions:
  User:
    type: object
    required:
      - email
    properties:
      email:
        type: string
  UserRegister:
    allOf:
      - $ref: '#/definitions/User'
    type: object
    required:
      - password
    properties:
      password:
        type: string

the required property password is not found when i call:

final Model model = swagger.getDefinitions().get("UserRegister");
final ComposedModel composedModel = (ComposedModel) model;

composedModel.getRequired(); // empty or null

This issue is affecting swagger-codegen reported at swagger-api/swagger-codegen#7058