[ENH] Coalesce fix

[samukweku]

Please describe the changes proposed in the pull request:

• coalesce only needs bfill

**This PR improves coalesce **

PR Checklist

Please ensure that you have done the following:

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

2. If you're not on the contributors list, add yourself to AUTHORS.rst.

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

There will be automatic checks run on the PR. These include:

• 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-1042--pyjanitor.netlify.app

[codecov]

Codecov Report

Merging #1042 (e819bed) into dev (88f5479) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev    #1042   +/-   ##
=======================================
  Coverage   97.68%   97.68%           
=======================================
  Files          77       77           
  Lines        2888     2888           
=======================================
  Hits         2821     2821           
  Misses         67       67           

[ericmjl]

LGTM, thank you @samukweku!