Fix array-based className handling to avoid comma separation in child elements

[szymonchudy]

Hey there! First, thanks so much for all the work required to build and maintain this library.

When integrating it into my blog, I noticed that class names were not being applied correctly. Specifically, when child elements receive an array of class names (e.g., ["hljs-title", "function_"]), React currently renders them as a comma-separated string ("hljs-title,function_"), which brakes CSS. This issue can be seen on the provided demo page as well:

This PR ensures that any className arrays are joined with spaces ("hljs-title function_") instead.

Changes

1. mapChild function checks if props.className is an array, and if so, joins it with a space.
2. The expected behavior is covered with a separate test. All existing tests pass without regression.

[rexxars]

Oops. Thanks!