Fix deprecation warning for assert_nil in ClassMethodTest

[floehopper]

$ ruby -v
ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]

$ bundle list | grep minitest
  * minitest (5.10.3)

$ bundle exec ruby -Itest test/unit/class_method_test.rb -n test_should_return_mock_for_stubbee
Run options: -n test_should_return_mock_for_stubbee --seed 24035

# Running:

DEPRECATED: Use assert_nil if expecting nil from test/unit/class_method_test.rb:185. This will fail in Minitest 6.
.

Finished in 0.002643s, 378.2878 runs/s, 378.2878 assertions/s.

1 runs, 1 assertions, 0 failures, 0 errors, 0 skips