Fix case-sensitive categories will override each other.

Let's say I have 2 categories `Git` and `git`.

For category links, they were generated by `category.to_url` so they will link to the same url `categories/git`

But in `plugins/category_generator.rb`, they were treated like different categories, so one of them will be overridden by the other. I think we should treat these as the same category. After all, they're generated to the same folder, and linked to the same url.