Fix for #6263 (replaces PR #6518)

## Issue summary

Elements of type `Zend\Form\ElementInterface` cannot be added directly to a Collection object due to the latter's setting of an object prototype on the supplied element, which is only defined on `Zend\Form\FieldsetInterface`.

## Fix

Handle `Zend\Form\ElementInterface` instances separately; call `setValue` instead of `setObject`.

## Ack
Fix and reproducing code snippet contributed by @SteveTalbot