Ensure asset metadata is updated when file name isn't changed

Currently, when an asset is updated with a file which has the same name as the
previous file, the metadata (specifically the MD5 hexdigest) doesn't get
updated. There is a guard statement in the S3 uploader that prevents assets
with and unchanged MD5 hexdigest) from uploading.

This is due to a condition added in #1258 that was intended to quieten some
errors we were seeing at the time. This same issue was addressed in #1263.

app/models/asset.rb

@@ -76,7 +76,7 @@ class Asset
 
   mount_uploader :file, AssetUploader
 
-  before_save :store_metadata, unless: :uploaded?, if: -> { changes.include?(:file) }
+  before_save :store_metadata, unless: :uploaded?
   after_save :schedule_virus_scan
   after_save :update_indirect_replacements_on_publish
   after_save :backpropagate_replacement

spec/models/asset_spec.rb

@@ -1105,6 +1105,19 @@
         expect(asset.save).to be_truthy
       end
     end
+
+    context "when the file name is unchanged" do
+      let(:saved_asset) { FactoryBot.create(:asset) }
+      let(:md5_hexdigest) { saved_asset.md5_hexdigest }
+      let(:original_filename) { saved_asset.file.send(:original_filename) }
+
+      it "updates the md5 hexdigest" do
+        asset = described_class.find(saved_asset.id) # find to clear memoization
+        asset.update!(file: load_fixture_file("lorem.txt", named: original_filename))
+
+        expect(asset.md5_hexdigest).not_to eq md5_hexdigest
+      end
+    end
   end
 
   describe "#replacement" do