title: mvc dropdownlistfor is not marked as required but is still required
body: <p>I have a dropdownlistfor in cshtml file:</p>
  
  <pre><code>var kategorie_wlasna = new SelectList(
    (from z in Model.Kategoria
      where !formReadOnly || z.Id == Model.KategoriaWlasnaId 
        select z), 
    "Id", 
    "Nazwa");
  ...
  @Html.DropDownListFor(
    model =&gt; model.KategoriaWlasnaId, 
    kategorie_wlasna,
    "----",
    htmlClassDropDownListDef)
  </code></pre>
  
  <p>In my viewModel I have the property without any annotations as Required:</p>
  
  <pre><code>public long KategoriaWlasnaId { get; set; }
  </code></pre>
  
  <p>But the field is still Required. In the browser I get:</p>
  
  <pre><code>&lt;select class="input-validation-error form_object1" data-val="true" data-val-number="The field KategoriaWlasnaId must be a number." data-val-required="The KategoriaWlasnaId field is required." id="KategoriaWlasnaId" name="KategoriaWlasnaId"&gt;
    &lt;option value=""&gt;-----&lt;/option&gt;
    &lt;option value="1227"&gt;Wykroczenie&lt;/option&gt;
    &lt;option value="1228"&gt;Przestępstwo&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>What am I missing?</p>