title: HttpUtility.ParseQueryString does not allow to add duplicate key
body: <p>I have Web API endpoint which expects a list of enum values. I am using HttpUtility.ParseQueryString to generate the query string where I want to pass multiple key-value pair in the query string as below,</p>
  
  <pre><code>../api/getdata?myEnum=One&amp;myEnum=Two&amp;myEnum=Three
  </code></pre>
  
  <p>But, the ParseQueryString is creating the query string url as below,</p>
  
  <pre><code>../api/getdata?myEnum=One%2cTwo%2cThree
  </code></pre>
  
  <p>Here is my code,</p>
  
  <pre><code>var queryString = System.Web.HttpUtility.ParseQueryString(string.Empty);
  queryString.Add("myEnum", "One");
  queryString.Add("myEnum", "Two");
  queryString.Add("myEnum", "Three");
  </code></pre>
  
  <p>How can I create the expected query string url with duplicate keys.</p>
  
  <pre><code>../api/getdata?myEnum=One&amp;myEnum=Two&amp;myEnum=Three
  </code></pre>