Trouble adding request headers in controller spec

controller.rb

class ApiController < ApplicationController

  def index
    # request.headers always equals {}
    if request.headers['Authorization'] == 'HELLO'
      render text: 'SUCCESS', status: :ok
    else
      render text: 'FAILURE', status: :unauthorized
  end

end

spec.rb

require 'spec_helper'

describe ApiController do
  render_views
  
  it 'should pass the authorization header' do
    request.headers['Authorization'] = 'HELLO'
    get :index, { format: :json }
    
    expect(response.status).to eq(200)
  end
end