Check return value of ObjectSpace.define_finalizer

ObjectSpace.define_finalizer returns an array of `[0, finalizer]`. When
the finalizer is not defined, it returns the finalizer that was passed
in. Otherwise, it returns the originally defined finalizer.

core/objectspace/define_finalizer_spec.rb

@@ -179,6 +179,20 @@ def finalizer(zelf)
     ruby_exe(code).should == "ok"
   end
 
+  it "returns the defined finalizer" do
+    obj = Object.new
+    p = proc { |id| }
+    p2 = p.dup
+
+    ret = ObjectSpace.define_finalizer(obj, p)
+    ret.should == [0, p]
+    ret[1].should.equal?(p)
+
+    ret = ObjectSpace.define_finalizer(obj, p2)
+    ret.should == [0, p]
+    ret[1].should.equal?(p)
+  end
+
   ruby_version_is "3.1" do
     describe "when $VERBOSE is not nil" do
       it "warns if an exception is raised in finalizer" do