Fix loss keys of named class in `Html::addCssClass()` when class in passed options is string

[vjik]

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌
Fixed issues

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (db526a8) 100.00% compared to head (22bd1c0) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #171 +/- ## =========================================== Coverage 100.00% 100.00% - Complexity 759 760 +1 =========================================== Files 84 84 Lines 2052 2053 +1 =========================================== + Hits 2052 2053 +1 ``` | [Files](https://app.codecov.io/gh/yiisoft/html/pull/171?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft) | Coverage Δ | | |---|---|---| | [src/Html.php](https://app.codecov.io/gh/yiisoft/html/pull/171?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yiisoft#diff-c3JjL0h0bWwucGhw) | `100.00% <100.00%> (ø)` | |

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[what-the-diff[bot]]

PR Summary

• Bug Fix in Html::addCssClass() method A bug that caused the loss of keys of named classes when a string was passed in the options has been fixed. This ensures that the HTML elements will have the correct CSS styles applied to them at all times.

• Improvement in Html::addCssClass() method The Html::addCssClass() method has been improved to better handle the merging of CSS classes. This means that the system can now efficiently and accurately manage multiple CSS styles that are being merged together.

• Increased Test Coverage The HtmlTest class has been expanded to include additional test cases for the Html::addCssClass() method. These new test scenarios improve our system's reliability by checking if the updates work correctly in more diverse situations.