determineTypeOfCollectionElement does not work for field in superclass

[robert-bor]

Fields in a superclass are not checked for their generic type.

class A {
   public Set<String> roles;
}

class B extends A {
}

When working on an instance of B, the generic type of the Set in A cannot be determined.

Also make sure to check the code in SetCollectionHandler.create. It now calls isAssignableFrom with a nullpointer. Make sure this does not happen.