title: How to add attribute without value
body: <p>I am generating HTML textbox through the html helper and TagBuilder.</p>
  
  <p></p>
  
  <p>we have the method TagBuilder.Attributes.Add("key","value")</p>
  
  <p>but for HTML5 required attribute does not need value to be passed, so if i pass empty string then the output with value of required = ""
  </p>
  
  <p>So how do i add required attribute without passing the value?</p>
  
  <pre><code> public static IHtmlString AppTextBox(this HtmlHelper helper, string model)
      {
          var input = new TagBuilder("input");
          input.Attributes.Add("class", "form-control");
          input.Attributes.Add("ng-model", model);
  
          input.Attributes.Add("required","");
  
          return new MvcHtmlString(input.ToString(TagRenderMode.Normal));
      }
  </code></pre>