don't do extra redirect in LDB popup window

fixes #6636

there seems to be a race condition where this redirect will sometimes
happen before the LDB notices the params telling it to go into
fullscreen lockdown mode. so we just avoid the redirect, which isn't
necessary since we didn't make a post request anyway.

test plan: in the LDB, go to a quiz that requires the LDB, ensure that
it goes fullscreen.

Change-Id: Idf94ae56ce5e695f51fe75da9d98611f60164666
Reviewed-on: https://gerrit.instructure.com/7564
Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>

app/controllers/quizzes_controller.rb

@@ -472,7 +472,13 @@ class QuizzesController < ApplicationController
       @submission = @quiz.generate_submission(user_code, !!preview)
     end
     if quiz_submission_active?
-      redirect_to named_context_url(@context, 'context_quiz_take_url', @quiz.id)
+      if request.get?
+        # currently, the only way to start_quiz! with a get request is to use the LDB
+        take_quiz
+      else
+        # redirect to avoid refresh issues
+        redirect_to named_context_url(@context, 'context_quiz_take_url', @quiz.id)
+      end
     else
       flash[:error] = t('errors.no_more_attempts', "You have no quiz attempts left") unless @just_graded
       redirect_to named_context_url(@context, :context_quiz_url, @quiz)