#204 - Fix CachedTransformer, SlugifyTransformer and TypeSetterTransformer edge cases

[njoubert-cleverage]

Description

Fixes #204.

Three transformers handle edge cases badly:

• cached rejects any non-string input with a TypeError, before key_transformers can convert it to a string, while the documentation says that a non-string key value just skips the cache.
• slugify accepts an invalid transliterator identifier, then fails on the first value with Call to a member function transliterate() on null.
• type_setter has a TransformerException branch for settype() returning false that can never run in PHP 8: invalid types are already rejected by setAllowedValues, and settype() never returns false for the allowed types.

This PR:

• CachedTransformer::generateCacheKey() takes a mixed $value: a key value that is still not a string after key_transformers skips the cache, as documented.
• The slugify transliterator normalizer throws an InvalidOptionsException (with the intl error message) when \Transliterator::create() returns null, so the error shows up when the options are resolved.
• Remove the dead TransformerException branch from TypeSetterTransformer::transform().
• Update the reference documentation, add unit tests.

The new regression tests fail on main and pass with this fix. PHPUnit, PHPStan, PHP-CS-Fixer and Rector pass.

Requirements

• Documentation updates
  • Reference
  • Cookbooks
  • Changelog
• Unit tests

Breaking changes

None for configurations. A subclass of CachedTransformer overriding generateCacheKey() with a string $value parameter must widen it to mixed.

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