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.

Author: eregon

lib/json/truffle_ruby/generator.rb

@@ -413,7 +413,7 @@ def generate(obj, anIO = nil)
               buf << obj.to_json(self)
             end
           when Integer
-            buf << String(obj)
+            buf << obj.to_s
           when Symbol
             if @strict
               fast_serialize_string(obj.name, buf)
@@ -652,9 +652,7 @@ def json_transform(state)
 
         module Integer
           # Returns a JSON string representation for this Integer number.
-          def to_json(*)
-            String(self)
-          end
+          def to_json(*) to_s end
         end
 
         module Float

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