title: Swagger generates string for DateTime
body: <p>I have this action method:</p>
  
  <pre><code>[HttpGet]
  public async Task&lt;ActionResult&lt;Agent[]&gt;&gt; Get(DateTime? lastModified = null) {}
  </code></pre>
  
  <p>I can see that the following swagger json is generated using Swashbuckle.AspNetCore v4.0.1:</p>
  
  <pre><code>"parameters":[{"name":"lastModified","in":"query","required":false,"type":"string","format":"date-time"}]
  </code></pre>
  
  <p>Why is the type <code>string</code>?
  Shouldn't it be <code>DateTime</code>?</p>