Use Array(ActiveSupport::ToJsonWithActiveSupportEncoder)#to_json and Hash(ActiveSupport::ToJsonWithActiveSupportEncoder)#to_json for Ruby 2.8.0

This pull request addresses failures at https://buildkite.com/rails/rails/builds/70219#79d96882-6c51-4854-8cab-28f50ac8bca1
According to https://bugs.ruby-lang.org/issues/16973 This is an expected change in Ruby.
These failures has been addressed by changing the order of prepend as suggested.

```diff
% git diff
diff --git a/activesupport/test/json/encoding_test.rb b/activesupport/test/json/encoding_test.rb
index 30a3b8e5a0..1328041bf7 100644
--- a/activesupport/test/json/encoding_test.rb
+++ b/activesupport/test/json/encoding_test.rb
@@ -186,6 +186,8 @@ def test_hash_should_pass_encoding_options_to_children_in_to_json
         country: "UK"
       }
     }
+    p person.method(:to_json)
+    pp person.class.ancestors
     json = person.to_json only: [:address, :city]

     assert_equal(%({"address":{"city":"London"}}), json)
@@ -287,6 +289,8 @@ def test_array_to_json_should_not_keep_options_around
     f.bar = "world"

     array = [f, { "foo" => "other_foo", "test" => "other_test" }]
+    p array.method(:to_json)
+    pp array.class.ancestors
     assert_equal([{ "foo" => "hello", "bar" => "world" },
                   { "foo" => "other_foo", "test" => "other_test" }], ActiveSupport::JSON.decode(array.to_json))
   end
%
```

* Ruby 2.8.0 without this fix uses `Array(JSON::Ext::Generator::GeneratorMethods::Array)#to_json`, which should use `Array(ActiveSupport::ToJsonWithActiveSupportEncoder)#to_json`

```
% bin/test test/json/encoding_test.rb -n test_array_to_json_should_not_keep_options_around
Run options: -n test_array_to_json_should_not_keep_options_around --seed 33311

[Array,
 JSON::Ext::Generator::GeneratorMethods::Array,
 ActiveSupport::ToJsonWithActiveSupportEncoder,
 Enumerable,
 ActiveSupport::ToJsonWithActiveSupportEncoder,
 Object,
 JSON::Ext::Generator::GeneratorMethods::Object,
 ActiveSupport::Tryable,
 Kernel,
 BasicObject]
F

Failure:
TestJSONEncoding#test_array_to_json_should_not_keep_options_around [/Users/yahonda/src/github.com/rails/rails/activesupport/test/json/encoding_test.rb:294]:
--- expected
+++ actual
@@ -1 +1 @@
-[{"foo"=>"hello", "bar"=>"world"}, {"foo"=>"other_foo", "test"=>"other_test"}]
+["#<TestJSONEncoding::CustomWithOptions:0xXXXXXX>", {"foo"=>"other_foo", "test"=>"other_test"}]

bin/test test/json/encoding_test.rb:286

Finished in 0.015486s, 64.5745 runs/s, 64.5745 assertions/s.
1 runs, 1 assertions, 1 failures, 0 errors, 0 skips
%
```

* Ruby 2.8.0 with this fix uses `Array(ActiveSupport::ToJsonWithActiveSupportEncoder)#to_json`

```
% bin/test test/json/encoding_test.rb -n test_array_to_json_should_not_keep_options_around
Run options: -n test_array_to_json_should_not_keep_options_around --seed 12193

[ActiveSupport::ToJsonWithActiveSupportEncoder,
 Array,
 JSON::Ext::Generator::GeneratorMethods::Array,
 ActiveSupport::ToJsonWithActiveSupportEncoder,
 Enumerable,
 ActiveSupport::ToJsonWithActiveSupportEncoder,
 Object,
 JSON::Ext::Generator::GeneratorMethods::Object,
 ActiveSupport::Tryable,
 Kernel,
 BasicObject]
.

Finished in 0.008070s, 123.9157 runs/s, 123.9157 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips
%
```

* Ruby 2.8.0 without this fix uses `Hash(JSON::Ext::Generator::GeneratorMethods::Hash)#to_json`, which should use `Hash(ActiveSupport::ToJsonWithActiveSupportEncoder)#to_json`

```
% bin/test test/json/encoding_test.rb -n test_hash_should_pass_encoding_options_to_children_in_to_json

Run options: -n test_hash_should_pass_encoding_options_to_children_in_to_json --seed 18064

[Hash,
 JSON::Ext::Generator::GeneratorMethods::Hash,
 ActiveSupport::ToJsonWithActiveSupportEncoder,
 Enumerable,
 ActiveSupport::ToJsonWithActiveSupportEncoder,
 Object,
 JSON::Ext::Generator::GeneratorMethods::Object,
 ActiveSupport::Tryable,
 Kernel,
 BasicObject]
F

Failure:
TestJSONEncoding#test_hash_should_pass_encoding_options_to_children_in_to_json [/Users/yahonda/src/github.com/rails/rails/activesupport/test/json/encoding_test.rb:193]:
--- expected
+++ actual
@@ -1 +1 @@
-"{\"address\":{\"city\":\"London\"}}"
+"{\"name\":\"John\",\"address\":{\"city\":\"London\",\"country\":\"UK\"}}"

bin/test test/json/encoding_test.rb:181

Finished in 0.015009s, 66.6267 runs/s, 66.6267 assertions/s.
1 runs, 1 assertions, 1 failures, 0 errors, 0 skips
%
```

* Ruby 2.8.0 with this fix uses `Hash(ActiveSupport::ToJsonWithActiveSupportEncoder)#to_json`

```
% bin/test test/json/encoding_test.rb -n test_hash_should_pass_encoding_options_to_children_in_to_json

Run options: -n test_hash_should_pass_encoding_options_to_children_in_to_json --seed 56794

[ActiveSupport::ToJsonWithActiveSupportEncoder,
 Hash,
 JSON::Ext::Generator::GeneratorMethods::Hash,
 ActiveSupport::ToJsonWithActiveSupportEncoder,
 Enumerable,
 ActiveSupport::ToJsonWithActiveSupportEncoder,
 Object,
 JSON::Ext::Generator::GeneratorMethods::Object,
 ActiveSupport::Tryable,
 Kernel,
 BasicObject]
.

Finished in 0.007434s, 134.5171 runs/s, 134.5171 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips
%
```

Refer
https://github.com/ruby/ruby/pull/3181
https://github.com/ruby/ruby/pull/2936
https://bugs.ruby-lang.org/issues/9573
https://github.com/rails/rails/pull/19413

activesupport/lib/active_support/core_ext/object/json.rb

@@ -45,7 +45,7 @@ module ActiveSupport
   end
 end
 
-[Object, Array, FalseClass, Float, Hash, Integer, NilClass, String, TrueClass, Enumerable].reverse_each do |klass|
+[Enumerable, Object, Array, FalseClass, Float, Hash, Integer, NilClass, String, TrueClass].reverse_each do |klass|
   klass.prepend(ActiveSupport::ToJsonWithActiveSupportEncoder)
 end