sendMessageStream: in preview should should return a stream and populate history when startChat is passed no request obj failed

Note: #532 was also for this test, but it was closed more than 10 days ago. So, I didn't mark it flaky. ---- commit: 0fd5e490eb471e9c423b6d88a50662b2fa3baf83 buildURL: [Build Status](https://source.cloud.google.com/results/invocations/3a757f13-8eba-46e7-85b7-5f40ba596e66), [Sponge](http://sponge2/3a757f13-8eba-46e7-85b7-5f40ba596e66) status: failed