Add a default parameter for get_reason_phrase helper, defaults to Unknown.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code docstring.
• Add an entry in CHANGES.md summarizing the change and linking to the issue and your username.
• Add *Version Changed* or *Version Added* note in any relevant docs and docstring.
• Run pytest and tox, no tests failed.