Router cascade boolean fix

According to @ujifgc in https://github.com/padrino/padrino-framework/pull/1471#issuecomment-26972057, it is allowed for `app.cascade` to have boolean values
I've noticed this issue when working with a mounted Grape API app which (by default) returns a boolean value for `app.cascade` call.

Enclosed in PR you will find the missing test and the fix.