Fixes ->form()->openTage($form) usage

Makes it possible to use ->openTag to render the form using correct layout classes.

Examples:

<?php echo $this->form()->openTag($this->customerForm); ?>
<?php echo $this->form(null, 'inline')->openTag($this->customerForm); ?>
Merge pull request #72 from maurice2k/patch-1

Fixes ->form()->openTag($form) usage