Monkey patch ActiveSupport to bring back to_json unicode character encoding. There is a better way if you have control over the to_json call, use JSON.generate(object, :ascii_only => true). Read more @ http://omegadelta.net/2013/04/28/changes-to-how-rails-3-2-13-and-4-0-encodes-unicode-in-json/

json_escape.rb

module ActiveSupport
  module JSON
    module Encoding
      class << self
      
      # from https://github.com/rails/rails/blob/v3.2.12/activesupport/lib/active_support/json/encoding.rb#L121
      def escape(string)
        if string.respond_to?(:force_encoding)
          string = string.encode(::Encoding::UTF_8, :undef => :replace).force_encoding(::Encoding::BINARY)
        end
        json = string.
          gsub(escape_regex) { |s| ESCAPED_CHARS[s] }.
          gsub(/([\xC0-\xDF][\x80-\xBF]|
                 [\xE0-\xEF][\x80-\xBF]{2}|
                 [\xF0-\xF7][\x80-\xBF]{3})+/nx) { |s|
          s.unpack("U*").pack("n*").unpack("H*")[0].gsub(/.{4}/n, '\\\\u\&')
        }

        ## alternate encoding from https://github.com/rails/rails/issues/3727
        #json = string.
        #  gsub(escape_regex) { |s| ESCAPED_CHARS[s] }.
        #  gsub(/([\xC0-\xDF][\x80-\xBF]|
        #         [\xE0-\xEF][\x80-\xBF]{2}|
        #         [\xF0-\xF7][\x80-\xBF]{3})+/nx) { |s|
        #  s.unpack("U*").pack("N*").unpack("H*")[0].gsub(/.{4}/n, '\\\\u\&')
        #}

        json = %("#{json}")
        json.force_encoding(::Encoding::UTF_8) if json.respond_to?(:force_encoding)
        json
        end
      end
    end
  end
end