Update rest of tests to include JSONAPI::MEDIA_TYPE in Accept header

Author: jerelmiller

test/integration/requests/namespaced_model_test.rb

@@ -6,7 +6,7 @@ def setup
   end
 
   def test_get_flat_posts
-    get '/flat_posts'
+    get '/flat_posts', headers: { 'Accept' => JSONAPI::MEDIA_TYPE }
     assert_equal 200, status
     assert_equal "flat_posts", json_response["data"].first["type"]
   end