Accept responses with no etag header

If the server does not provide an etag header, treat all responses as a
cache miss. This should allow 3rd party gem servers to more easily
provide compact_index results. /cc @mperham

See also sidekiq/sidekiq#4158.

bundler/lib/bundler/compact_index_client/updater.rb

@@ -66,16 +66,16 @@ def update(local_path, remote_path, retrying = nil)
             end
           end
 
-          response_etag = (response["ETag"] || "").gsub(%r{\AW/}, "")
-          if etag_for(local_temp_path) == response_etag
+          etag = (response["ETag"] || "").gsub(%r{\AW/}, "")
+          if etag.length.zero? || etag_for(local_temp_path) == etag
             SharedHelpers.filesystem_access(local_path) do
               FileUtils.mv(local_temp_path, local_path)
             end
             return nil
           end
 
           if retrying
-            raise MisMatchedChecksumError.new(remote_path, response_etag, etag_for(local_temp_path))
+            raise MisMatchedChecksumError.new(remote_path, etag, etag_for(local_temp_path))
           end
 
           update(local_path, remote_path, :retrying)

bundler/spec/bundler/compact_index_client/updater_spec.rb

@@ -16,15 +16,13 @@
 
     let(:response) { double(:response, :body => "") }
 
-    it "MisMatchedChecksumError is raised" do
+    it "treats the response as an update" do
       # Twice: #update retries on failure
-      expect(response).to receive(:[]).with("Content-Encoding").twice { "" }
-      expect(response).to receive(:[]).with("ETag").twice { nil }
-      expect(fetcher).to receive(:call).twice { response }
+      expect(response).to receive(:[]).with("Content-Encoding") { "" }
+      expect(response).to receive(:[]).with("ETag") { nil }
+      expect(fetcher).to receive(:call) { response }
 
-      expect do
-        updater.update(local_path, remote_path)
-      end.to raise_error(Bundler::CompactIndexClient::Updater::MisMatchedChecksumError)
+      updater.update(local_path, remote_path)
     end
   end