When a route is mounted inside a resources block, it's automatically
prefixed, so a following code:
resources :users do
mount Blog::Engine => '/blog'
will generate a user_blog path helper.
In order to access engine helpers, we also use "mounted_helpers", a list
of helpers associated with each mounted engine, so a path to blog's post
can be generated using user_blog.post_path(user, post).
The problem I'm fixing here is that mount used a raw :as option, without
taking nestings into account. As a result, blog was added to a route set
as a `user_blog`, but helper was generated for just `blog`.
This commit applies the proper logic for defining a helper for a mounted
engine nested in resources or resource block.