Fix double encoding of html diff output in syndicated feeds and subscrition emails

Resolves, FS#2738 (feeds)[1] & FS#2748 (emails)[2] and brings both feed.php & inc/subscription.php into line with the changes made to inc/DifferenceEngine.php in PR#179.

ref. 
[1] https://bugs.dokuwiki.org/index.php?do=details&task_id=2738
[2] https://bugs.dokuwiki.org/index.php?do=details&task_id=2748