fix a bug in defined?(super) which returns the incorrect value if use in included module

Test Code:
----
module Stuff
  def foo
    puts "In Stuff#foo"
    if defined?(super)
      puts "Calling super"
      super
    else
      puts "No super"
    end
  end
end

module OtherStuff
  def foo
    puts "In OtherStuff#foo"
    p defined?(super)
  end
end

class NoSuper
  include Stuff
end
NoSuper.new.foo

class WithSuper
  include OtherStuff
  include Stuff
end
WithSuper.new.foo

Result
----
$ macruby t.rb
In Stuff#foo
Calling super
/Volumes/Mac/Users/watson/tmp/t.rb:6:in `foo': super: no superclass method `foo' for #<NoSuper:0x4001a33a0> (NoMethodError)
	from /Volumes/Mac/Users/watson/tmp/t.rb:23:in `<main>'

Expected
----
$ ruby t.rb
In Stuff#foo
No super
In Stuff#foo
Calling super
In OtherStuff#foo
nil