[jss-plugin-cache] don't set the selector when rule was cached before

Currently, when the cache plugin returns a cached rule from onCreateRule, we still generate a new class name and set the selector [here](https://github.com/cssinjs/jss/blob/92a53e4da49749b46b300eeae7bc6fadcc006931/packages/jss/src/RuleList.js#L77).

The problem with this is that in case the selector was modified before by a custom plugin, we are removing the modification.

Problems:
- we still need to get back the class name, currently className is not saved on the StyleRule instance, because its hard to keep it consistently with .selector property.

Eventually we should still save somewhere the 'generatedClassName' and be able to use it to register a rule comming from the cache.

Start at packages/jss/src/RuleList.js around line 77 and trace how the cache plugin's onCreateRule result is handled, including the StyleRule selector and className state. Done means a cached rule preserves selector changes made by a custom plugin while its generated class name remains available for registration.

Written by the indexing model from the issue text.