[ENH] explicit default parameter for case_when

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

~~- standalone function (if_else) that can be imported for use in pandas.assign~~

• explicit default parameter added

This PR resolves #1159 .

PR Checklist

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1. [x] PR in from a fork off your branch. Do not PR from <your_username>:dev, but rather from <your_username>:<feature-branch_name>.

2. [x] If you're not on the contributors list, add yourself to AUTHORS.md.

3. [ ] Add a line to CHANGELOG.md under the latest version header (i.e. the one that is "on deck") describing the contribution.
   • Do use some discretion here; if there are multiple PRs that are related, keep them in a single line.

Automatic checks

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• Building a preview of the docs on Netlify
• Automatically linting the code
• Making sure the code is documented
• Making sure that all tests are passed
• Making sure that code coverage doesn't go down.

Relevant Reviewers

Please tag maintainers to review.

• @ericmjl

[ericmjl]

🚀 Deployed on https://deploy-preview-1165--pyjanitor.netlify.app

[codecov[bot]]

Codecov Report

Merging #1165 (dc12acc) into dev (4eaef97) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1165      +/-   ##
==========================================
- Coverage   97.96%   97.96%   -0.01%     
==========================================
  Files          76       76              
  Lines        3395     3387       -8     
==========================================
- Hits         3326     3318       -8     
  Misses         69       69              

[Zeroto521]

Two threads (https://github.com/pyjanitor-devs/pyjanitor/runs/8295244245, https://github.com/pyjanitor-devs/pyjanitor/runs/8295231459) CI failed due to new PR #1158.

case_when testcases raised some errors like

=========================== short test summary info ============================
ERROR tests/functions/test_case_when.py - hypothesis.errors.InvalidArgument: ...
ERROR tests/functions/test_case_when.py - hypothesis.errors.InvalidArgument: ...
ERROR tests/functions/test_case_when.py - hypothesis.errors.InvalidArgument: ...
ERROR tests/functions/test_case_when.py - hypothesis.errors.InvalidArgument: ...

[ericmjl]

@thatlittleboy would you like to do the honors here? a -0.01% decrease in coverage is marginal, we can handle it :smile:

[thatlittleboy]

yep okies, merging