Case insensitive shell completions for Choice(..., case_sensitive=False)

[felixnorden]

This PR adds the logic and a test for checking case insensitive completions for Choice instances which have the argument case_sensitive=False.

What changed was having the Choice class' shell_complete-method take self.case_sensitive into account when matching on the provided instance choices by using temporary string transforms on the incomplete argument and self.choices into all lower case when self.case_sensitive=False.

• Fixes shell completion is case sensitive, even with click.Choice(case_sensitive=False) #1692

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.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed. (NOTE: No test results changed with this contribution, there was 1 failing test though)

[bgschiller]

Thanks for the quick PR! It might be worth adding a test for the opposite behavior: that case sensitive choice don't autocomplete when the case doesn't match.

[felixnorden]

Sorry for the delay, I have added a test case for this as well and the tests are passing as expected :)

[davidism]

Refactored to avoid using an inner function. Refactored test to use paremtrize and test choices both cases.