Let any class response type be passed to the serializer

This fixes #13 by letting the Serializer throw a `ReflectionException`.

It also has the added benefit of allowing the `responseClass` value to be a Serializer array form (eg a JSON response without a root element can be deserialized by setting it to `array<Vendor\\MyBundle\\Entity\\Person>`).

(Note that `ArrayCollection<Vendor\\MyBundle\\Entity\\Person>` isn't supported until https://github.com/schmittjoh/serializer/issues/9 is fixed.)