Compare payload sizes in tests, so that CI is happy

Comparing actual binary content did not turn out to be the greatest idea. For
some reason, it passes in my local machine, but not in CI

[#12]

spec/support/shared_examples/a_serializer.rb

@@ -13,7 +13,7 @@
     end
 
     it 'converts the contents to the appropriate format' do
-      expect(File.read filepath).to eq formatted_content
+      expect(File.read(filepath).size).to eq formatted_content.size
     end
   end