Add tests for the behavior of JSON.generate with base types subclasses

Ref: ruby#674
Ref: ruby#668

The behavior on such case it quite unclear, the goal here is to
figure out whatever was the behavior on Cext version of `json 2.7.0`
and get all implementations to converge.

We can then decide to make them all behave differently if we so wish.

test/json/json_generator_test.rb

@@ -536,6 +536,72 @@ def test_to_json_called_with_state_object
     assert_instance_of JSON::State, argument
   end
 
+  module CustomToJSON
+    def to_json(*)
+      %{"#{self.class.name}#to_json"}
+    end
+  end
+
+  module CustomToS
+    def to_s
+      "#{self.class.name}#to_s"
+    end
+  end
+
+  class ArrayWithToJSON < Array
+    include CustomToJSON
+  end
+
+  def test_array_subclass_with_to_json
+    assert_equal '"JSONGeneratorTest::ArrayWithToJSON#to_json"', JSON.generate(ArrayWithToJSON.new)
+    assert_equal '{"[]":1}', JSON.generate(ArrayWithToJSON.new => 1)
+  end
+
+  class ArrayWithToS < Array
+    include CustomToS
+  end
+
+  def test_array_subclass_with_to_s
+    assert_equal '[]', JSON.generate(ArrayWithToS.new)
+    assert_equal '{"JSONGeneratorTest::ArrayWithToS#to_s":1}', JSON.generate(ArrayWithToS.new => 1)
+  end
+
+  class HashWithToJSON < Hash
+    include CustomToJSON
+  end
+
+  def test_hash_subclass_with_to_json
+    assert_equal '"JSONGeneratorTest::HashWithToJSON#to_json"', JSON.generate(HashWithToJSON.new)
+    assert_equal '{"{}":1}', JSON.generate(HashWithToJSON.new => 1)
+  end
+
+  class HashWithToS < Hash
+    include CustomToS
+  end
+
+  def test_hash_subclass_with_to_s
+    assert_equal '{}', JSON.generate(HashWithToS.new)
+    assert_equal '{"JSONGeneratorTest::HashWithToS#to_s":1}', JSON.generate(HashWithToS.new => 1)
+  end
+
+  class StringWithToJSON < String
+    include CustomToJSON
+  end
+
+  def test_string_subclass_with_to_json
+    assert_equal '"JSONGeneratorTest::StringWithToJSON#to_json"', JSON.generate(StringWithToJSON.new)
+    assert_equal '{"":1}', JSON.generate(StringWithToJSON.new => 1)
+  end
+
+  class StringWithToS < String
+    include CustomToS
+  end
+
+  def test_string_subclass_with_to_s
+    assert_equal '""', JSON.generate(StringWithToS.new)
+    assert_equal '{"JSONGeneratorTest::StringWithToS#to_s":1}', JSON.generate(StringWithToS.new => 1)
+  end
+
   if defined?(JSON::Ext::Generator) and RUBY_PLATFORM != "java"
     def test_valid_utf8_in_different_encoding
       utf8_string = "€™"