Fix stream spec: and_return needs a return value.

[Disallow and_return without arguments: rspec/rspec-mocks@e1c9a3e](https://github.com/rspec/rspec-mocks/commit/e1c9a3e21435b2159fcdbcf1e6aebd1554d2f890)

But I am not sure what value should return, I tried `true` and everything passed.