ObjectHydrator: fix entity namespaces.

# Object Hydrator: fix entity namespaces

If you are using Entity Namespace aliases, the ObjectHydrator will throw a notice for an undefined index of your entity namespace.

## Problem
The problem lies in the fact that the prepare() method uses the "className", used in the aliasMap (where you use the namespace alias) to store the local ClassMetadata cache. Though, in a later stage the actual namespace is being used to find this same item.

## Fix
I've changed the way this ClassMetadata cache is built. It now uses the full Entity namespace.