Add case normalization to day of week parsing

[jadchaar]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• [x] 🧪 Added tests for changed code.
• [x] 🛠️ All tests pass when run locally (run tox or make test to find out!).
• [x] 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• [ ] 📚 Updated documentation for changed code.
• [x] ⏩ Code is up-to-date with the master branch.

If you have any questions about your code changes or any of the points above, please submit your questions along with the pull request and we will try our best to help!

Description of Changes

Fixed a case sensitivity issue reported in #851, and added missing asserts! Closes: https://github.com/arrow-py/arrow/issues/851

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (e22c531) to head (f8041ce). Report is 184 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #852 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 9 9 Lines 1797 1799 +2 Branches 308 310 +2 ========================================= + Hits 1797 1799 +2 ```

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