Merge pull request #27366 from utilum/avoid_fixnum_warning

Fix Fixnum deprecated warning in Ruby 2.4+

activesupport/test/core_ext/duration_test.rb

@@ -21,7 +21,7 @@ class << k; undef_method :== end
   end
 
   def test_instance_of
-    assert 1.minute.instance_of?(Fixnum)
+    assert 1.minute.instance_of?(1.class)
     assert 2.days.instance_of?(ActiveSupport::Duration)
     assert !3.second.instance_of?(Numeric)
   end