set the user id headers before and after a request

... so that it will be set on both login and logout requests.

test plan:
 * Open network inspector and login. Verify that on the login request,
   X-Canvas-User-Id is set.
 * Log out.
 * Verify that on the logout request, X-Canvas-User-Id is still set.

Change-Id: Ib849185cc3e289ae288e1ed2fe0ebae48f26de97
Reviewed-on: https://gerrit.instructure.com/12221
Tested-by: Zach Wily <zach@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>

app/controllers/application_controller.rb

@@ -41,6 +41,9 @@ class ApplicationController < ActionController::Base
   after_filter :log_page_view
   after_filter :discard_flash_if_xhr
   after_filter :cache_buster
+  # Yes, we're calling this before and after so that we get the user id logged
+  # on events that log someone in and log someone out.
+  after_filter :set_user_id_header
   before_filter :fix_xhr_requests
   before_filter :init_body_classes
   before_filter :set_ua_header
@@ -116,8 +119,8 @@ class ApplicationController < ActionController::Base
   end
 
   def set_user_id_header
-    headers['X-Canvas-User-Id'] = @current_user.global_id.to_s if @current_user
-    headers['X-Canvas-Real-User-Id'] = @real_current_user.global_id.to_s if @real_current_user
+    headers['X-Canvas-User-Id'] ||= @current_user.global_id.to_s if @current_user
+    headers['X-Canvas-Real-User-Id'] ||= @real_current_user.global_id.to_s if @real_current_user
   end
 
   # make things requested from jQuery go to the "format.js" part of the "respond_to do |format|" block

app/controllers/pseudonym_sessions_controller.rb

@@ -362,6 +362,8 @@ class PseudonymSessionsController < ApplicationController
   end
 
   def successful_login(user, pseudonym)
+    @current_user = user
+    @current_pseudonym = pseudonym
     respond_to do |format|
       flash[:notice] = t 'notices.login_success', "Login successful." unless flash[:error]
       if session[:oauth2]