Update and add tests in array_ext_test.rb

- Fix `test_to_with_instruct` typo to `test_to_xml_with_instruct`

- Rename `test_to_xml` to `test_to_xml_with_hash_elements` to make test
name more specific.

- Add `test_to_xml_with_non_hash_elements` and
`test_to_xml_with_non_hash_different_type_elements`

  `to_xml` behaves different when containing elements are same and
  different types.

- Add `test_to_xml_with_indent_set`

Above is the commit message, and when I'm reading `array_ext_test.rb`, I see a [FIXME](https://github.com/rails/rails/blob/master/activesupport/test/core_ext/array_ext_test.rb#L6) tag.
I'm just wondering what does it exactly mean, maybe I can help to fix it.