Remove respond_with_content_type from assignments_controller_test.rb

Removed these final 2 warnings from the functional tests:

```
DEPRECATION WARNING: The respond_with_content_type matcher is deprecated and will be removed in 2.0. (called from __bind_1376274264_585530 at /Users/danielstjules/GitHub/Markus/test/functional/assignments_controller_test.rb:540)
DEPRECATION WARNING: The respond_with_content_type matcher is deprecated and will be removed in 2.0. (called from __bind_1376274264_601772 at /Users/danielstjules/GitHub/Markus/test/functional/assignments_controller_test.rb:546)
```