Add tests for small chunk downloads via the streaming interface

This also tests that it's possible to customise the chunk size on
individual adapters.

spec/bucket_store_integration_spec.rb

@@ -99,6 +99,22 @@
         expect(rebuilt_large_file.size).to eq(large_file_content.size)
         expect(rebuilt_large_file).to eq(large_file_content)
       end
+
+      it "allows downloads of individual small chunks" do
+        described_class.
+          for("#{base_bucket_uri}/large.txt").
+          upload!("1234567890")
+
+        chunks = described_class.for("#{base_bucket_uri}/large.txt").
+          stream.
+          download(chunk_size: 1).
+          to_a
+
+        expect(chunks.size).to eq(10)
+        expect(chunks.map { |_meta, chunk| chunk }).to match_array(
+          %w[1 2 3 4 5 6 7 8 9 0],
+        )
+      end
     end
   end