rspec: Websockets Sample returns HTML failed

Note: #1566 was also for this test, but it was closed more than 10 days ago. So, I didn't mark it flaky. ---- commit: a743f4030b03b56d5872466b4a075c07db8090df buildURL: [Build Status](https://source.cloud.google.com/results/invocations/9d8f3429-04b5-4391-87eb-a9e5408f5cba), [Sponge](http://sponge2/9d8f3429-04b5-4391-87eb-a9e5408f5cba) status: failed