Improve receiver and sender test coverage

[gavv]

See coverage here:

• https://coveralls.io/builds/58298808/source?filename=roc%2Freceiver.go
• https://coveralls.io/builds/58298808/source?filename=roc%2Fsender.go

Cover the following branches:

• Receiver:

  • OpenReceiver: nil context
  • OpenReceiver: closed context

• Sender:

  • OpenSender: nil context
  • OpenSender: closed context

[gavv]

See also #43, #44.