title: How can I change the header of WordPress 'retrieve_password' email?
body: <p>I noticed WordPress give us some filters to customize the password reset request email like <code>wp_mail_from</code>, <code>retrieve_password_title</code> and <code>retrieve_password_message</code> and these last two are executed in the <code>retrieve_password</code> function at <code>wp-login.php</code> as following:</p>
  
  <pre><code>$title = apply_filters( 'retrieve_password_title', $title );
  $message = apply_filters( 'retrieve_password_message', $message, $key );
  
  if ( $message &amp;&amp; !wp_mail( $user_email, wp_specialchars_decode( $title ), $message ) )
      wp_die( __('The e-mail could not be sent.') . "&lt;br /&gt;\n" . __('Possible reason: your host may have disabled the mail() function.') );
  
  return true;
  </code></pre>
  
  <p>That's cool but I also realized the <code>wp_mail</code> function does not use any header which make me believe everything should be override with filters.</p>
  
  <p>Is there a filter to override the headers? Specifically <code>Content-Type</code>? And where would be the best place to register them?</p>