Fix an imprecise spec for Dash#replace

The check here doesn't make sense because it's testing the behavior of
another method. The best way to check identity is but checking the
`#object_id` of an object - thus, this check actually does what the test
says it does.

spec/hashie/dash_spec.rb

@@ -344,7 +344,7 @@ class KeepingMash < Hashie::Mash
     before { subject.replace(first_name: 'Cain') }
 
     it 'return self' do
-      expect(subject.replace(email: 'bar').to_hash).to eq(email: 'bar', count: 0)
+      expect(subject.replace(email: 'bar').object_id).to eq subject.object_id
     end
 
     it 'sets all specified keys to their corresponding values' do