Revert "Remove unicode character encoding from ActiveSupport::JSON.encode"

This reverts commit 815a9431ab61376a7e8e1bdff21f87bc557992f8.

Conflicts:
	activesupport/test/json/encoding_test.rb

Reason: This was causing a regression where the resulting string is always
returning UTF-8. Also it changes the behavior of this method on a stable release.

Fixes #9498.