PartialLoop helper: prevent convert traversable model to array recursively

I consider when we pass a model as traversable to partial loop, we don't need to convert it to array recursively.
For example, when we pass a form collection (of fieldsets) to partial loop as the model, if we converted it to array recursively -> fieldsets will converted to array too, in view script we can't access the fieldset through object key.
Merge branch 'patch-1' of https://github.com/tranba/zf2 into patch-1