title: Swagger: take one or more values from enum
body: <p>I am writing an OpenAPI (Swagger) definition where a query parameter can take none, or N values, like this:</p>
  
  <pre><code>/path?sort=field1,field2
  </code></pre>
  
  <p>How can I write this in OpenAPI YAML?</p>
  
  <p>I tried the following, but it does not produce the expected result:</p>
  
  <pre class="lang-yaml prettyprint-override"><code>- name: sort
    in: query
    schema:
      type: string
      enum: [field1,field2,field3]
    allowEmptyValue: true
    required: false
    description: Sort the results by attributes. (See http://jsonapi.org/format/1.1/#fetching-sorting)
  </code></pre>