title: sendGrid set headers/content type
body: <p>I'm looking to use sendGrid to create an outlook meeting request.</p>
  
  <p>I need to be able to set the content type (header) to something specific, however does anyone know how I can do this? I presume I am to use the x-smtpapi, whilst accepting a json string.</p>
  
  <p>Here is what I currently have.</p>
  
  <pre><code>$json_string = array(
  'Content-Type' =&gt; array(
      'text/calendar; Content-Disposition: inline; charset=utf-8;\r\n',
      'text/plain;charset=\"utf-8\"\r\n'
  ));
  </code></pre>
  
  <p>Many thanks</p>