Avoid method redefinition warnings in test_broken_bignum

Author: byroot

test/json/json_generator_test.rb

@@ -493,20 +493,20 @@ def foo.to_h
     assert_equal '2', state.indent
   end
 
-  if defined?(JSON::Ext::Generator) and RUBY_PLATFORM != "java"
-    def test_broken_bignum # [ruby-core:38867]
-      bignum = 1 << 64
-      original_to_s = bignum.class.instance_method(:to_s)
-      bignum.class.class_eval do
-        def to_s
-        end
-      end
-      assert_raise(TypeError) do
-        JSON::Ext::Generator::State.new.generate(bignum)
+  def test_broken_bignum # [Bug #5173]
+    bignum = 1 << 64
+    original_to_s = bignum.class.instance_method(:to_s)
+    bignum.class.class_eval do
+      def to_s
+        nil
       end
-    ensure
-      bignum.class.define_method(:to_s, original_to_s) if original_to_s
+      alias_method :to_s, :to_s
+    end
+    assert_raise(TypeError) do
+      JSON::Ext::Generator::State.new.generate(bignum)
     end
+  ensure
+    bignum.class.define_method(:to_s, original_to_s) if original_to_s
   end
 
   def test_hash_likeness_set_symbol