DRYed up "@foo=self.class.foo unless @foo" statements

Using the example from the existing code:

  CHECKSUM_TYPES.each do |type|
    if !instance_variable_defined?("@#{type}")
      class_value = self.class.send(type)
      instance_variable_set("@#{type}", class_value) if class_value
    end
  end

I extracted that block into a method 'set_instance_variable' which I
then used in all places where this behavior was being used.