Fix AS::Cache version upgrading bug

[josh]

Regression from e3a746b

/cc @dhh @bdurand

[dhh]

Would be nice to have a test for this case.

[josh]

I really have no idea how to reproduce it with a test case.

It just bombs with an existing tmp/cache running sprockets after that commit.

[josh]

Dealing with different cache versions seems like a huge pita.

[tenderlove]

diff --git a/activesupport/test/caching_test.rb b/activesupport/test/caching_test.rb
index febf0ee..9f76f4c 100644
--- a/activesupport/test/caching_test.rb
+++ b/activesupport/test/caching_test.rb
@@ -3,6 +3,20 @@ require 'abstract_unit'
 require 'active_support/cache'

 class CacheKeyTest < ActiveSupport::TestCase
+  def test_entry_legacy_optional_ivars
+    legacy = Class.new(ActiveSupport::Cache::Entry) do
+      def initialize(value, options = {})
+        @value = value
+        @expires_in = nil
+        @created_at = nil
+        super
+      end
+    end
+
+    entry = legacy.new 'foo'
+    assert_equal 'foo', entry.value
+  end
+
   def test_expand_cache_key
     assert_equal '1/2/true', ActiveSupport::Cache.expand_cache_key([1, '2', true])
     assert_equal 'name/1/2/true', ActiveSupport::Cache.expand_cache_key([1, '2', true], :name)

Dealing with different cache versions seems like a huge pita.

We should probably add a version ivar to objects that can be serialized similar to what Rubgems does for gem specifications. That would make updating easier.