nacking messages when context canceled

[roblaszczak]

Motivation / Background

Fix for: #555

Detail

Alternative approaches considered (if applicable)

Checklist

The resources of our team are limited. There are a couple of things that you can do to help us merge your PR faster:

• I wrote tests for the changes.
• All tests are passing.
  • If you are testing a Pub/Sub, you can start Docker with make up.
  • You can start with make test_short for a quick check.
  • If you want to run all tests, use make test.
• Code has no breaking changes.
• (If applicable) documentation on watermill.io is updated.
  • Documentation is built in the github.com/ThreeDotsLabs/watermill/docs.
  • You can find development instructions in the DEVELOP.md.