[ENH] Complete dict fix

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• Fix bug where complete fails if the args is just a dictionary of length 1, and the values is an instance of pd.Index.

This PR resolves #947 .

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

[codecov]

Codecov Report

Merging #948 (d398baa) into dev (34935eb) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #948      +/-   ##
==========================================
- Coverage   95.96%   95.96%   -0.01%     
==========================================
  Files          19       19              
  Lines        2530     2527       -3     
==========================================
- Hits         2428     2425       -3     
  Misses        102      102              

[ericmjl]

The changes are small enough to review easily. Thank you, @samukweku, for handling this!