Improve code coverage of Stream

- Wrote additional tests, to cover all code paths.
- Fixed several issues encountered.
Improved test coverage of Response

- Covered all code paths
Improve Request test coverage

- Covered all code paths
Improve test coverage of Middleware

- Ensure tests exist for Object::handle()-style middleware
Improve code coverage of Next

- Added a test that chains several routes sharing common paths, to
  ensure resetSlash() and resetPath() get called.
Improve coverage of Utils::getArity()
Improve AbstractMessage coverage

- Cover all code paths
Merge branch 'hotfix/code-coverage'

Improve test coverage