Delete FIXME annotation in dispatch/mapper_test.rb

Delete `FIXME` in `actionpack/test/dispatch/mapper_test.rb` because it's fixed.

Author: koya1616

actionpack/test/dispatch/mapper_test.rb

@@ -127,7 +127,6 @@ def test_map_more_slashes
         fakeset = FakeSet.new
         mapper = Mapper.new fakeset
 
-        # FIXME: is this a desired behavior?
         mapper.get "/one/two/", to: "posts#index", as: :main
         assert_equal "/one/two(.:format)", fakeset.asts.first.to_s
       end