fix: support uppercase custom props in toHaveStyle

[depoulo]

What:

support uppercase custom props in toHaveStyle

fixes #551

Why:

CSS custom properties are case sensitive

How:

don't lowercase custom property names before comparing them

Checklist:

• [ ] Documentation N/A
• [x] Tests
• [ ] Updated Type Definitions N/A
• [x] Ready to be merged

--

please squash and merge

related: #148, #164

[codecov[bot]]

Codecov Report

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

Comparison is base (4ae0231) 100.00% compared to head (90296fc) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #552 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 27 27 Lines 664 668 +4 Branches 251 252 +1 ========================================= + Hits 664 668 +4 ```

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[github-actions[bot]]

:tada: This PR is included in version 6.1.5 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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