Fix DRbServer#any_to_s

My previous fix in d0ed935d5bf8c3fce9800742a36e44fb7f63dda4 was
not correct, as pointed out by cremno on GitHub.

This simplifies things by just using Kernel#to_s.  Also switch to
bind_call(obj) instead of bind(obj).call for better performance.

lib/drb/drb.rb

@@ -1606,15 +1606,9 @@ module DRb
     # Coerce an object to a string, providing our own representation if
     # to_s is not defined for the object.
     def any_to_s(obj)
-      obj.to_s + ":#{obj.class}"
+      "#{obj}:#{obj.class}"
     rescue
-      case obj
+      Kernel.instance_method(:to_s).bind_call(obj)
-      when Object
-        klass = obj.class
-      else
-        klass = Kernel.instance_method(:class).bind(obj).call
-      end
-      sprintf("#<%s:0x%dx>", klass, obj.__id__)
     end
 
     # Check that a method is callable via dRuby.

test/drb/test_drb.rb

@@ -327,6 +327,20 @@ class TestBug4409 < Test::Unit::TestCase
   end
 end
 
+class TestDRbAnyToS < Test::Unit::TestCase
+  class BO < BasicObject
+  end
+
+  def test_any_to_s
+    server = DRb::DRbServer.new('druby://:0')
+    server.singleton_class.send(:public, :any_to_s)
+    assert_equal("foo:String", server.any_to_s("foo"))
+    assert_match(/\A#<DRbTests::TestDRbAnyToS::BO:0x[0-9a-f]+>\z/, server.any_to_s(BO.new))
+    server.stop_service
+    server.thread.join
+  end
+end
+
 class TestDRbTCP < Test::Unit::TestCase
   def test_immediate_close
     server = DRb::DRbServer.new('druby://:0')