attr_accessor_with_default with collections

I started digging into this when I stumbled across this SO question: http://stackoverflow.com/questions/3481406
To give an example of the unexpected behavior:

class Container
  attr_accessor_with_default :foo, [0, 0]
end

# This works as expected
c = Container.new
c.foo = ["foo", "bar"]  # => ["foo", "bar"]
Container.new.foo       # => [0, 0]
c.foo[0] = 0            # => [0, "bar"]
Container.new.foo       # => [0, 0]

# This changes the class default value
c2 = Container.new
c2.foo                  # => [0, 0]
c2.foo[0] = "problem!"  # => ["problem!", 0]
Container.new.foo       # => ["problem!", 0]

I'll be submitting a patch shortly.