[BUGFIX] htmlArea RTE: CSS class names are lowercase in WebKit

Problem: See https://bugs.webkit.org/show_bug.cgi?id=55619
Solution: Add doctype to the iframe source

Change-Id: Id045e469d8ff5e37fab03e4c96238317d6e8e967
Resolves: #32408
Releases: 4.5, 4.6, 4.7
Reviewed-on: http://review.typo3.org/8951
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland