title: Swagger UI don't display enum values in models
body: <p>I'm facing a little problem using Swagger UI and I'm not sure yet wheter is it a bug or not...</p>
  
  <p>I've described my API using Swagger Editor and I have a method that contains an enum parameter.</p>
  
  <p>Now, the problem: in the Swagger-UI documentation generated, in the 'model' tab, I have an empty definition of the 'messageType' object :(  I want to display the allowed values (BRAND, VISITOR, COMMENT and RESPONSE)</p>
  
  <p>A code snippet to reproduce:</p>
  
  <pre><code>swagger: '2.0'
  info:
    version: 1.0.0-SNAPSHOT
    title: foo
    description: foo
  host: dev.fr
  basePath: /base
  schemes:
    - http
    - https
  consumes:
    - application/json
  produces:
    - application/json
  
  paths:
    /social/message:
      post:
        operationId: create
        responses:
          '201':
            schema:
              $ref: '#/definitions/message'
          'default':
            description: Default error response
  
  definitions:
    message:
      required:
        - title
        - messageType
      properties:
        title:
          type: string
        messageType:
          $ref: '#/definitions/messageType'
  
    messageType:
      enum:
        - COMMENT
        - RESPONSE
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/JJOpO.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/JJOpO.png" alt="screen capture"></a></p>
  
  <p>Any idea ? I am doing it wrong ? Is it a bug ?
  Many thanks :)</p>