Fix missing inheritance of disabled tags

[dylanahsmith]

Problem

I noticed that when we subclass the Liquid::Render tag, it doesn't inherit the disabled tags or set the @disabled_tags instance variable on the class. This results in an internal error when the tag subclassing the Liquid::Render tag is used without the subclass also calling disable_tags "include". Also, if the subclass tried to disable another tag, it could easily re-enable the tag disabled in the subclass.

Solution

Use the same approach as rails' class_attribute of defining a method when disable_tags is called to override the parent's disabled_tags, without the method needing to be called to inherit the parent class' method.