Missing required second argument in array_diff

The $whiteListAttributes variable is missing on the comparison function. 
This pr corrects previous https://github.com/joomla/joomla-cms/pull/2575
Test using assertXmlStringEqualsXmlString instead of simple string comparison
Remove unnecessary php behavior check.
Merge pull request #2577 from mariopro/patch-12

Missing required second argument in array_diff
Merge pull request #2620 from okonomiyaki3000/fix_travis

Test using assertXmlStringEqualsXmlString instead of simple string comparison