Forms that are not explicitly inheriting from SubForm can still work

I'm using Multipage in combination with:
https://github.com/Emagister/zend-form-decorators-bootstrap

Their Form base classes don't inherit from SubForm. Simply setting the $_isArray property on the forms is enough, though. Only problem is that Zend Multipage Forms explicitly checks for SubForm. This minor patch also allows regular forms, as long as the isArray is set to true.