Add AbstractCookieJar class.

Eventually this will be the superclass of all the chained jars.

actionpack/lib/action_dispatch/middleware/cookies.rb

@@ -412,7 +412,7 @@ def write_cookie?(cookie)
         end
     end
 
-    class PermanentCookieJar #:nodoc:
+    class AbstractCookieJar # :nodoc:
       include ChainedCookieJars
 
       def initialize(parent_jar)
@@ -422,7 +422,9 @@ def initialize(parent_jar)
       def [](name)
         @parent_jar[name.to_s]
       end
+    end
 
+    class PermanentCookieJar < AbstractCookieJar # :nodoc:
       def []=(name, options)
         if options.is_a?(Hash)
           options.symbolize_keys!