Improve test coverage

[osiegmar]

As commented in #64: My current concerns are about the networking stuff (like #62). The corner cases like timeouts or TLS handshakes needs some more testing. And those tests could identify that a different implementation (NIO) is needed. NIO usually comes with higher complexity, hence I'd only use it with a good reason. This reason might be performance – a test has to proof it. The reason might be this socket timeout issue – a test would be needed as well.