Updating TestSession to access with indifference

The following Rails code failed (with a `KeyError` exception) under
test:

```ruby
class ApplicationController < ActionController::Base
  def user_strategy
    # At this point:
    # ```ruby
    # session == {
    #   "user_strategy"=>"email",
    #   "user_identifying_value"=>"hello@world.com"
    # }
    # ```
    if session.key?(:user_strategy)
      session.fetch(:user_strategy)
    end
  end
end
```

When I checked the session's keys (`session.keys`), I got an array of
strings. If I accessed `session[:user_strategy]` I got the expected
`'email'` value. However if I used `session.fetch(:user_strategy)` I
got a `KeyError` exception.

This appears to be a Rails 4.2.4 regression (as the code works under
Rails 4.2.3).

Closes #21383

1 files changed, 2 insertions, 2 deletions

diff --git a/actionpack/lib/action_controller/test_case.rb b/actionpack/lib/action_controller/test_case.rb
index 103986e44a..39cbc0cd70 100644
--- a/actionpack/lib/action_controller/test_case.rb
+++ b/actionpack/lib/action_controller/test_case.rb
@@ -174,8 +174,8 @@ module ActionController
       clear
     end
 
-    def fetch(*args, &block)
-      @data.fetch(*args, &block)
+    def fetch(key, *args, &block)
+      @data.fetch(key.to_s, *args, &block)
     end
 
     private