Add tests for ReactiveCaching key cleanup

- ReactiveCaching should attempt deletion of value keys which do not
  have an associated alive key

spec/models/concerns/reactive_caching_spec.rb

@@ -196,6 +196,12 @@ describe ReactiveCaching, :use_clean_rails_memory_store_caching do
         2.times { instance.exclusively_update_reactive_cache! }
       end
 
+      it 'does not delete the value key' do
+        expect(Rails.cache).to receive(:delete).with(cache_key).never
+
+        go!
+      end
+
       context 'and #calculate_reactive_cache raises an exception' do
         before do
           stub_reactive_cache(instance, "preexisting")
@@ -223,6 +229,12 @@ describe ReactiveCaching, :use_clean_rails_memory_store_caching do
 
         go!
       end
+
+      it 'deletes the value key' do
+        expect(Rails.cache).to receive(:delete).with(cache_key).once
+
+        go!
+      end
     end
 
     context 'when the lease is already taken' do