fix: Use ContentRange in the right way.

[jeffpeng3]

Summary

According to werkzeug's documentation, end is not included in the range, so end should not be reduced by one.

Changes:

Changed end - 1 to end and modified test at the same time.

Issues:

• fixes #331

Checklist:

• [x] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [x] Add or update relevant docs, in the docs folder and in code.
• [x] Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [x] Add .. versionchanged:: entries in any relevant code docs.
• [x] Run pre-commit hooks and fix any issues.
• [x] Run pytest and tox, no tests failed.