title: asp.net core Taghelper merge with existing class attribute
body: <p>I have a textbox with default class name, then wanna able to merge with new pass in class name.</p>
  <p>I try created custom tag helper as below:</p>
  <pre><code>[HtmlTargetElement(&quot;input-form-control&quot;)]
  public class FormControlTextBox : TagHelper
  {
      private string ExtraClasses;
  
      public override void Process(TagHelperContext context, TagHelperOutput output)
      {
          output.Attributes.SetAttribute(&quot;class&quot;, $&quot;form-control {ExtraClasses}&quot;);
      }
  }
  </code></pre>
  <p>How do I make it as my expected result as below:</p>
  <pre><code>&lt;input class=&quot;form-control extra-class&quot;&gt;&lt;/input&gt;
  </code></pre>
  <p>Any better way to archive this?</p>