json_generator_test.rb: no Bignum

* test/json/json_generator_test.rb (test_broken_bignum): get rid
  of use of Bignum, obsolete name.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56126 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/json/json_generator_test.rb

@@ -277,12 +277,13 @@ EOT
   if defined?(JSON::Ext::Generator)
     def test_broken_bignum # [ruby-core:38867]
       pid = fork do
-        Bignum.class_eval do
+        x = 1 << 64
+        x.class.class_eval do
           def to_s
           end
         end
         begin
-          JSON::Ext::Generator::State.new.generate(1<<64)
+          JSON::Ext::Generator::State.new.generate(x)
           exit 1
         rescue TypeError
           exit 0