Fix handleLogout ignoring wreply override

This is the fix for https://groups.google.com/forum/#!topic/simplesamlphp/Ga8dbqNc92M
Per http://msdn.microsoft.com/en-us/library/bb608217.aspx
wreply is in fact supposed to replace the normal behavior, and is optional.
Tested to work correctly in our Dev, GoDaddy QA  will be verifying it as well, but I see no issue.