BugFix Issue #248 - Excel2007 does not correctly mark rows as hidden

In the main loop of load method in Excel 2007
if you print a var_dump($col["hidden"])
you'll get this:

object(SimpleXMLElement)#13032 (1) {
  [0]=>
  string(1) "1"
}

So, the "self::boolean" method, does not work properly.
I fixed that method including the "is_object" check.