fix to catch error when it retrieves polymorphic association

closes #113

Author: eitoball

lib/comma/header_extractor.rb

@@ -45,7 +45,8 @@ def get_association_class(model_class, association)
 
       begin
         model_class.reflect_on_association(association)&.klass
-      rescue NameError
+      rescue ArgumentError, NameError
+        # Since Rails 5.2, ArgumentError is raised.
         nil
       end
     end

spec/comma/rails/active_record_spec.rb

@@ -144,8 +144,7 @@ def name
 
     describe 'github pull-request 83' do
       it 'should not raise NameError' do
-        expect { Picture.all.to_comma(:pr_83) }
-          .not_to raise_exception(NameError)
+        expect { Picture.all.to_comma(:pr_83) }.not_to raise_error
       end
     end
   end