rails/rails — pull request #12699 by drogus (2013-10-30 11:13:35+00:00)
Title: Fix mounting engines inside a resources block

From a commit message

```
When a route is mounted inside a resources block, it's automatically
prefixed, so a following code:

resources :users do
mount Blog::Engine => '/blog'
end

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.

(closes #8533)
```