Include the namespace of the associated class

If the associated class is `System::Build`, the previous code would
tell us that the name was `Build`. This code gets the right name.

Example taken from
thoughtbot#1978 (comment),
which I reproduced locally.

lib/administrate/field/associative.rb

@@ -12,7 +12,7 @@ def self.associated_class(resource_class, attr)
       end
 
       def self.associated_class_name(resource_class, attr)
-        reflection(resource_class, attr).class_name
+        associated_class(resource_class, attr).name
       end
 
       def self.reflection(resource_class, attr)