title: Shallow routes inside a namespace with :path param not working
body: <p>This routes setting</p>
  
  <pre><code>namespace :api, path: nil, except: [:new, :edit] do
    resources :blogs do
      resources :comments
    end
  end
  </code></pre>
  
  <p>gave me this and it's ok.</p>
  
  <pre><code>GET    /blogs/:blog_id/comments(.:format)     api/comments#index
  POST   /blogs/:blog_id/comments(.:format)     api/comments#create
  GET    /blogs/:blog_id/comments/:id(.:format) api/comments#show
  PATCH  /blogs/:blog_id/comments/:id(.:format) api/comments#update
  DELETE /blogs/:blog_id/comments/:id(.:format) api/comments#destroy
  
  GET    /blogs(.:format)                       api/blogs#index
  POST   /blogs(.:format)                       api/blogs#create
  GET    /blogs/:id(.:format)                   api/blogs#show
  PATCH  /blogs/:id(.:format)                   api/blogs#update
  DELETE /blogs/:id(.:format)                   api/blogs#destroy
  </code></pre>
  
  <p>But when I add "shallow: true" to the setting above</p>
  
  <pre><code>namespace :api, path: nil, except: [:new, :edit] do
    resources :blogs, shallow: true do
      resources :comments
    end
  end
  </code></pre>
  
  <p>an unwanted path '/api' comes up.</p>
  
  <pre><code>/api/blogs/:blog_id/comments(.:format) api/comments#index
  /api/blogs/:blog_id/comments(.:format) api/comments#create
  /api/comments/:id(.:format)            api/comments#show
  /api/comments/:id(.:format)            api/comments#update
  /api/comments/:id(.:format)            api/comments#destroy
  
  /blogs(.:format)                       api/blogs#index
  /blogs(.:format)                       api/blogs#create
  /api/blogs/:id(.:format)               api/blogs#show
  /api/blogs/:id(.:format)               api/blogs#update
  /api/blogs/:id(.:format)               api/blogs#destroy
  </code></pre>
  
  <p>Is this an expected behavior still in Rails 4?
  Should I write each resources respectively?</p>