Pure rack apps can be mounted with a name

This is a 4.2 regression from 4.1. In 4.1 you used to be able to name a mounted rack middleware. And the edgeapis still state it is possible.

See https://github.com/rails/rails/commit/9b15828b5c347395b42066a588c88e5eb4e72279#commitcomment-8764492

@rafaelfranca @tenderlove for review please.
Good point. Fixed.