Generate shallow paths for all children of shallow resources.

Prior to this commit shallow resources would only generate paths for
non-direct children (with a nested depth greater than 1).

Take the following routes file.

    resources :blogs do
      resources :posts, shallow: true do
        resources :comments do
          resources :tags
        end
      end
    end

This would generate shallow paths for `tags` nested under `posts`,
e.g `/posts/:id/tags/`, however it would not generate shallow paths
for `comments` nested under `posts`, e.g `/posts/:id/comments/new`.

This commit changes the behaviour of the route mapper so that it
generate paths for direct children of shallow resources, for example
if you take the previous routes file, this will now generate
shallow paths for `comments` nested under `posts`, .e.g
`posts/:id/comments/new`.

This was the behaviour in Rails `4.0.4` however this was broken in
@jcoglan's fix for another routes related issue[1].

This also fixes an issue[2] reported by @smdern.

[1] https://github.com/rails/rails/commit/d0e5963
[2] https://github.com/rails/rails/issues/15783

actionpack/CHANGELOG.md

+*   Generate shallow paths for all children of shallow resources.
+
+    Fixes #15783.
+
+    *Seb Jacobs*
+
 *   JSONP responses are now rendered with the `text/javascript` content type
     when rendering through a `respond_to` block.
 
@@ -177,5 +183,4 @@
 
     *Tony Wooster*
 
-
 Please check [4-1-stable](https://github.com/rails/rails/blob/4-1-stable/actionpack/CHANGELOG.md) for previous changes.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -1434,7 +1434,7 @@ def nested
           end
 
           with_scope_level(:nested) do
-            if shallow? && shallow_nesting_depth > 1
+            if shallow? && shallow_nesting_depth >= 1
               shallow_scope(parent_resource.nested_scope, nested_options) { yield }
             else
               scope(parent_resource.nested_scope, nested_options) { yield }

actionpack/test/dispatch/routing_test.rb

@@ -2022,6 +2022,28 @@ def test_shallow_deeply_nested_resources
     assert_equal '/blogs/1/posts/2/comments/new', new_blog_post_comment_path(:blog_id => 1, :post_id => 2)
   end
 
+  def test_direct_children_of_shallow_resources
+    draw do
+      resources :blogs do
+        resources :posts, shallow: true do
+          resources :comments
+        end
+      end
+    end
+
+    post '/posts/1/comments'
+    assert_equal 'comments#create', @response.body
+    assert_equal '/posts/1/comments', post_comments_path('1')
+
+    get '/posts/2/comments/new'
+    assert_equal 'comments#new', @response.body
+    assert_equal '/posts/2/comments/new', new_post_comment_path('2')
+
+    get '/posts/1/comments'
+    assert_equal 'comments#index', @response.body
+    assert_equal '/posts/1/comments', post_comments_path('1')
+  end
+
   def test_shallow_nested_resources_within_scope
     draw do
       scope '/hello' do