Change expected response code for add_content_ratings to 201

To address the change introduced in https://github.com/mozilla/zamboni/commit/3e1b7b4261154fc38079ce523b21c7f65b3f413d

Pinging @zalun for review/merge. This is not urgent as it's only a fix to the example app, a comment and some tests, but it should go into the codebase at some point.