[ruby/json] Keep Integer#to_json optimized and adapt the test

* Redefining Integer#to_s is bound to break many things,
  any non-segfault outcome should be acceptable.
  So adapt the test to just ensure it does not segfault.
* Also redefining Kernel#String would also break with the previous code.

ruby/json@ac0670b20b

Author: eregon

test/json/json_generator_test.rb

@@ -504,10 +504,14 @@ def to_s
       end
       alias_method :to_s, :to_s
     end
-    case RUBY_PLATFORM
-    when "java"
+    case RUBY_ENGINE
+    when "jruby"
       assert_equal bignum_to_s, JSON.generate(bignum)
-    else
+    when "truffleruby"
+      assert_raise(NoMethodError) do
+        JSON.generate(bignum)
+      end
+    when "ruby"
       assert_raise(TypeError) do
         JSON.generate(bignum)
       end