If the class is abstract, does the discriminator field/value really matter? The `$name` property is still referring to the abstract class, and we'd attempt to instantiate it below (likewise with #893, which uses the Reflection API to construct it in PHP 5.4+).

I've never seen an exception with `newInstance()` attempting to create an abstract class. Did you have a use case that triggered it? Otherwise, I would expect ODM to have safeguards in place to not invoke this method unless it knows exactly which concrete class it needs for hydration. That's not to say the exception isn't helpful -- I'm just curious if you actually encountered an error through this code path.