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.
I'm thinking now that it's probably a better idea to just use jesse_state:new/2 instead, the same way I do in check_disallow/3, since we don't really care about the resulting state.