delegate IIIF Manifest cache key version to presenter

app/presenters/hyrax/iiif_manifest_presenter.rb

@@ -133,6 +133,16 @@ def work_presenters
       member_presenters.select(&:work?)
     end
 
+    ##
+    # @note ideally, this value will be cheap to retrieve, and will reliably
+    #   change any time the manifest JSON will change. the current implementation
+    #   is more blunt than this, changing only when the work itself changes.
+    #
+    # @return [String] a string tag suitable for cache keys for this manifiest
+    def version
+      object.try(:modified_date)&.to_s || ''
+    end
+
     ##
     # An Ability-like object that gives `true` for all `can?` requests
     class NullAbility

app/services/hyrax/caching_iiif_manifest_builder.rb

@@ -51,7 +51,7 @@ def manifest_cache_key(presenter:)
     #
     # @return [String]
     def version_for(presenter)
-      presenter.etag
+      presenter.version
     end
   end
 end

spec/presenters/hyrax/iiif_manifest_presenter_spec.rb

@@ -209,5 +209,21 @@
       end
     end
   end
+
+  describe '#version' do
+    let(:work) { create(:work) }
+
+    it 'returns a string' do
+      expect(presenter.version).to be_a String
+    end
+
+    context 'when the work is unsaved' do
+      let(:work) { build(:monograph) }
+
+      it 'is still a string' do
+        expect(presenter.version).to be_a String
+      end
+    end
+  end
 end
 # rubocop:enable Style/BracesAroundHashParameters

spec/services/hyrax/caching_iiif_manifest_builder_spec.rb

@@ -11,7 +11,7 @@
     double(
       'Presenter',
       id: id,
-      etag: etag,
+      version: etag,
       work_presenters: [work_presenter],
       manifest_url: manifest_url,
       description: ["A Treatise on Coding in Samvera"],