title: Icalendar .ics not showing issue after adding html content
body: <p>I got this issue with icalendar, it doesn't show the attachment and the confirmation button when sent to gmail,yahoo, aoutlook.com.
  it was working before without html message and multipart/mixed header.
  now when i recieve the email it only show me the the text of the calendar, it doesn't recognize it.
      Content-type: multipart/mixed; boundary="982f8d486c3cbed0e28b314e7a433c3d"</p>
  
  <pre><code>--982f8d486c3cbed0e28b314e7a433c3d
  Content-Type: multipart/alternative; boundary="19f4b0e33c6f735ae6f858b1df7b72f2"
  
  --19f4b0e33c6f735ae6f858b1df7b72f2
  Content-Type: text/calendar; method=REQUEST; charset="iso-8859-1"
  Content-Transfer-Encoding: quoted-printable
  
  BEGIN:VCALENDAR
  VERSION:2.0
  PRODID:-//Testa//Tes//EN
  METHOD:REQUEST
  BEGIN:VEVENT
  UID:20150914T205515-998107994-domain.com
  DTSTAMP:20150914T205515
  DTSTART:20150915T195500Z
  DTEND:20150915T195500Z
  ORGANIZER;CN=soy:MAILTO:admin@adminm.org
  ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN=xxx.yyy@gmail.com:MAILTO:xxx.yyy@gmail.com
  LOCATION:hj
  SUMMARY:Rendez-vous 
  BEGIN:VALARM
  TRIGGER:-PT15M
  ACTION:DISPLAY
  DESCRIPTION:Reminder
  END:VALARM
  END:VEVENT
  END:VCALENDAR
  
  --19f4b0e33c6f735ae6f858b1df7b72f2--
  --982f8d486c3cbed0e28b314e7a433c3d
  Content-Type: application/ics; name="Rv-invite.ics"
  Content-Disposition: attachment; filename="Rv-invite.ics"
  Content-Transfer-Encoding: base64
  X-Mailer: Microsoft Office Outlook 12.0
  X-Mailer: CakePHP Email
  Date: Mon, 14 Sep 2015 20:55:16 +0200
  Message-ID: &lt;55f71814db484f8fa8923d8664ca69fc@localhost&gt;
  Subject: Rendez-vous 
  MIME-Version: 1.0
  Content-Type: text/html; charset=UTF-8
  Content-Transfer-Encoding: 8bit
  
  &lt;!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"&gt;
  &lt;html&gt;
  &lt;head&gt;
          &lt;title&gt;Emails/html&lt;/title&gt;
  &lt;/head&gt;
  &lt;body&gt;
  
  &lt;p&gt;Dear,&lt;/p&gt;
  
  &lt;p&gt;Mr xxx sent an appointment&lt;/p&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>thanks in advance.</p>