Fixes a TwitterStreamArtifact mapping issue with Streaming

These changes fix an issue with streaming. The unit test for streams had 2 problems:

 1. It wasn't ever terminating. I put in a cap for the max number of stream events.
 1. It wasn't properly serializing the data. I'm not exactly sure how hammock works, so forgive me if I solved the problem incorrectly. It looked like maybe the `RestResponseBase` was responsible for translating its data to the `TwitterStreamArtifact`, but that wasn't incredibly obvious to me.