title: plainToClass does not convert a Date to string
body: <p>According to <a href="https://github.com/typestack/class-transformer#%D1%81onverting-date-strings-into-date-objects" rel="nofollow noreferrer">the docs</a> a <code>Date</code> object should be converted to a <code>string</code>:</p>
  
  <blockquote>
    <p>Note, that dates will be converted to strings when you'll try to convert class object to plain object.</p>
  </blockquote>
  
  <p>My example-code with class-transformer <code>0.2.3</code> does not work as expected:</p>
  
  <pre><code>class TestDate {
    @Type(() =&gt; Date)
    aDate!: Date;
  }
  
    const testDate = new TestDate();
    testDate.aDate = new Date();
    const result: any = classToPlain(testDate);
    console.log(typeof result.aDate);
  </code></pre>
  
  <p>This prints <code>object</code> to the console, but I'd expect <code>string</code>.<br>
  What am I missing?</p>