Fix validation for array with set of types (refs #31)

This is a fix for #31 

Setting current_schema to the actual type being checked against fixes the 'no_extra_properties_allowed' issue mentioned. 

I'm still not happy about the actual error reporting because whatever you do, it would only report a "wrong_type". No matter if there are actual bad additional properties, missing properties or additional items.
Yes, agree, we shouldn't imply we're reusing anything here.