Test panic at 'called `Result::unwrap()` on an `Err` value: RpcFinished(None)'

[gengliqi]

There's some probability that many tests can panic at

'called Result::unwrap() on an Err value: RpcFinished(None)', src/libcore/result.rs:999:5

in v0.4.x branch test client_streaming::test_insecure https://travis-ci.org/pingcap/grpc-rs/jobs/582522697 test ping_pong::test_secure https://travis-ci.org/pingcap/grpc-rs/jobs/582522698 test empty_stream::test_insecure https://travis-ci.org/pingcap/grpc-rs/jobs/582659998