fix broken cache tests

activesupport/lib/active_support/cache.rb

@@ -591,9 +591,9 @@ def size
           when NilClass
             0
           when String
-            value.bytesize
+            @v.bytesize
           else
-            @s = Marshal.dump(value).bytesize
+            @s = Marshal.dump(@v).bytesize
           end
         end
       end

activesupport/test/caching_test.rb

@@ -616,7 +616,7 @@ def test_log_exception_when_cache_read_fails
 
 class MemoryStoreTest < ActiveSupport::TestCase
   def setup
-    @record_size = Marshal.dump("aaaaaaaaaa").bytesize
+    @record_size = ActiveSupport::Cache::Entry.new("aaaaaaaaaa").size
     @cache = ActiveSupport::Cache.lookup_store(:memory_store, :expires_in => 60, :size => @record_size * 10)
   end