rails: request.xhr? is not working in controller

class AlphaController < ApplicationController
  def home
    @microblogs = Microblog.all

    binding.pry

    if request.xhr?
        render :json => {
            :microblogs => @microblogs
        }
    end
  end
end

I wrote this action and used request.xhr? method to tell if the request is sent via ajax. But what I found in debugging is that the result returned by xhr? is always nil. A ruby method whose name ends with question mark is supposed to always return true or false, isn’t it?

I wonder if request.xhr? is deprecated in Rails 4.1.