title: show dateTime dataType in DataContract model on swagger UI via swashbuckle webapi 2 C#
body: <p>I am using a datacontract for the model and the datetime datamember is working properly, however swagger ui is showing the datatype as string instead of dateTime.</p>
  <p>datamember example:</p>
  <pre><code>[DataMember(EmitDefaultValue = false, Order = 6), XmlElement(Namespace = apiNameSpace, Order = 6)]
  public DateTime? StartDate { get; set; } = null;
  </code></pre>
  <p>swashbuckle swagger-ui model output:</p>
  <p>StartDate (string, optional):</p>