serialized :attribute fails when specified in STI subclass

When using STI, if a subclass specifies that an attribute is to be serialized, and that attribute was not specified as serialized in the base class, serialization of the attribute fails.

This bug will only occur in production, not in development mode due to the class reloading in development mode.

class Droid < ActiveRecord::Base

end

class R2D2 < Droid

serialize :secret_message

end

If the R2D2 class is loaded in production after the Droid class is loaded, then the serialization of the secret_message attribute will fail.

Patch and tests attached.