fix pluralization for words like "class"

I think `{ ~r/(ss)$/i, "\\1" }` may have been the in @pluralize section erroneously (the equivalent is not present in active support's plural inflections). It prevented words like "class" from being matched by `{ ~r/(x|ch|ss|sh)$/i, "\\1es" }` which caused them to be incorrectly pluralized.

Singularization for "classes" was not affected but I added tests for both singularization and pluralization to stick with the pattern.