Test that the model namespace isn't polluted.

The goal of ClassMethodsProxy is to avoid polluting the target's namespace,
but it was possible to do this by accident when calling `class_eval` before
ActiveSupport was completely loaded. This test ensures the namespace isn't
polluted, regardless of the load state of ActiveSupport.

elasticsearch-model/spec/elasticsearch/model/adapters/active_record/import_spec.rb

@@ -52,6 +52,10 @@
       it 'imports all documents' do
         expect(ImportArticle.search('*').results.total).to eq(10)
       end
+
+      it "does not pollute the model's namespace" do
+        expect(ImportArticle.methods).not_to include(:__transform)
+      end
     end
 
     context 'when batch size is specified' do