Fix SortedSet subclasses that override initialize

The first time SortedSet#initialize is called, it overwrites
itself, then recalls #initialize, which results in calling the
subclass's initialize, not the current initialize.

Just inline the default initialize behavior to avoid this issue.

No test for this as it can only be triggered the very first time
that SortedSet#initialize is called.

Fixes [Bug #15830]

lib/set.rb

@@ -804,7 +804,8 @@ class SortedSet < Set
 
   def initialize(*args, &block) # :nodoc:
     SortedSet.setup
-    initialize(*args, &block)
+    @keys = nil
+    super
   end
 end