Test that define_singleton_method uses public

In CRuby this logic happens to always be public because the target
class (the target object's singleton class) will almost never
match the caller's scope. Pending CRuby confirming that this is
the intended behavior and making it explicit so the caller's frame
is never used for define_singleton_method visibility.

core/kernel/define_singleton_method_spec.rb

@@ -96,4 +96,17 @@ def o.define(name)
       o.define(:foo) { raise "not used" }
     }.should raise_error(ArgumentError)
   end
+
+  it "always defines the method with public visibility" do
+    cls = Class.new
+    def cls.define(name, &block)
+      private
+      define_singleton_method(name, &block)
+    end
+
+    -> {
+      cls.define(:foo) { :ok }
+      cls.foo.should == :ok
+    }.should_not raise_error(NoMethodError)
+  end
 end