title: class-validator fails validation for IsDateString with 'must be ISOString but value logged is clearly an ISO String?
body: <p>The exact error: </p>
  
  <pre><code>    "value": "2019-10-09T10:36:40.791Z",
      "property": "createdAt",
      "children": [],
      "constraints": {
        "isDateString": "createdAt must be a ISOString"
      }
  </code></pre>
  
  <p>The field in the class:</p>
  
  <pre><code>    @prop({ default: new Date(Date.now()) })
      @IsDefined()
      @IsDateString()
      createdAt: Date;
  </code></pre>
  
  <p>getting the document coming from the db: </p>
  
  <pre><code> const order = await this.order.findOne({
        _id: body._id,
        clientId: user.clientId,
      });
  </code></pre>
  
  <p>calling the validator: </p>
  
  <pre><code>      let orderData = order.toJSON();
        let finalOrder = plainToClass(FinishedOrder, orderData);
        let validation = await validateOrReject(finalOrder);
  </code></pre>
  
  <p>I've also tried to call <code>order.toObject()</code> but it has the same effect.</p>