[BUGFIX] ValidatorResolver shouldn't ask class loader for simple types
The ValidatorResolver generates a Validator conjunction for a given class
if that class exists. Problem is that there is no check for simple types
before this, so a simple type will trigger the class loader (every time).
To avoid that we now check if the given type is a simple type.
Releases: master, 3.0, 2.3, 2.2