Add #dig to ActionDispatch::Request::Session

### Summary

The `session` object is not a real Hash but responds to many methods of Hash
such as `[]`, `[]`, `fetch`, `has_key?`.

Since Ruby 2.3, Hash also supports a `dig` method.

This commit adds a `dig` method to `ActionDispatch::Request::Session` with the
same behavior as `Hash#dig`.

This is useful if you store a hash in your session, such as:

```ruby
session[:user] = { id: 1, avatar_url: "http://example.org/nyancat.jpg" }
```

Then you can shorten your code from `session[:user][:avatar_url]` to `session.dig :user, :avatar_url`.

### Other Information

I cherry-picked a commit from https://github.com/rails/rails/pull/23864, and modify a bit.
The changes are below:

* Converts only the first key to a string adjust to the `fetch` method.
* Fixes a test case because we cannot use the indifferent access since ee5b621e2f8fde380ea4bc75b0b9d6f98499f511.

2 files changed, 20 insertions, 0 deletions

diff --git a/actionpack/lib/action_dispatch/request/session.rb b/actionpack/lib/action_dispatch/request/session.rb
index 000847e193..2ff651fc31 100644
--- a/actionpack/lib/action_dispatch/request/session.rb
+++ b/actionpack/lib/action_dispatch/request/session.rb
@@ -93,6 +93,14 @@ module ActionDispatch
         @delegate[key.to_s]
       end
 
+      # Returns the nested value specified by the sequence of key, returning
+      # nil if any intermediate step is nil.
+      def dig(*keys)
+        load_for_read!
+        keys = keys.map.with_index { |key, i| i.zero? ? key.to_s : key }
+        @delegate.dig(*keys)
+      end
+
       # Returns true if the session has the given key or false.
       def has_key?(key)
         load_for_read!
diff --git a/actionpack/test/dispatch/request/session_test.rb b/actionpack/test/dispatch/request/session_test.rb
index bf5a74e694..74da2fe7d3 100644
--- a/actionpack/test/dispatch/request/session_test.rb
+++ b/actionpack/test/dispatch/request/session_test.rb
@@ -118,6 +118,18 @@ module ActionDispatch
         end
       end
 
+      def test_dig
+        session = Session.create(store, req, {})
+        session["one"] = { "two" => "3" }
+
+        assert_equal "3", session.dig("one", "two")
+        assert_equal "3", session.dig(:one, "two")
+
+        assert_nil session.dig("three", "two")
+        assert_nil session.dig("one", "three")
+        assert_nil session.dig("one", :two)
+      end
+
       private
         def store
           Class.new {