title: Update existing header field in asterisk
body: <p>I am new to and working on asterisk. I have a question.
  How to extend a string to an existing header field in asterisk?
  For example, after calling: </p>
  
  <pre><code>add_header(req, "User_Note", "swim fast");
  </code></pre>
  
  <p>User_Note has value "swim fast". I want to add "run quickly" to the value of "User_Note". So "User_Note" will be "swim fast, run quickly".</p>