Make ActionDispatch::Request::Session#fetch behave like Hash#fetch

Session#fetch was mutating the session when given a default argument
and/or a block. Since Session duck-types as a Hash, it should behave
like one in these cases.

actionpack/lib/action_dispatch/request/session.rb

@@ -7,6 +7,9 @@ module ActionDispatch
       ENV_SESSION_KEY         = Rack::Session::Abstract::ENV_SESSION_KEY # :nodoc:
       ENV_SESSION_OPTIONS_KEY = Rack::Session::Abstract::ENV_SESSION_OPTIONS_KEY # :nodoc:
 
+      # Singleton object used to determine if an optional param wasn't specified
+      Unspecified = Object.new
+
       def self.create(store, env, default_options)
         session_was = find env
         session     = Request::Session.new(store, env)
@@ -127,15 +130,12 @@ module ActionDispatch
         @delegate.delete key.to_s
       end
 
-      def fetch(key, default=nil)
-        if self.key?(key)
-          self[key]
-        elsif default
-          self[key] = default
-        elsif block_given?
-          self[key] = yield(key)
+      def fetch(key, default=Unspecified, &block)
+        load_for_read!
+        if default == Unspecified
+          @delegate.fetch(key.to_s, &block)
         else
-          raise KeyError
+          @delegate.fetch(key.to_s, default, &block)
         end
       end
 

actionpack/test/dispatch/request/session_test.rb

@@ -68,13 +68,12 @@ module ActionDispatch
         assert_equal '1', session.fetch(:one)
 
         assert_equal '2', session.fetch(:two, '2')
-        assert_equal '2', session.fetch(:two)
+        assert_nil session.fetch(:two, nil)
 
         assert_equal 'three', session.fetch(:three) {|el| el.to_s }
-        assert_equal 'three', session.fetch(:three)
 
         assert_raise KeyError do
-          session.fetch(:four)
+          session.fetch(:three)
         end
       end