use `message` that specified in argument to error message

activesupport/lib/active_support/testing/assertions.rb

@@ -159,7 +159,7 @@ module ActiveSupport
           error = "#{message}.\n#{error}" if message
           assert_not_equal before, after, error
         else
-          message = "#{expression.inspect} didn't change to #{to}"
+          error = "#{expression.inspect} didn't change to #{to}"
           error = "#{message}.\n#{error}" if message
           assert to === after, error
         end

activesupport/test/test_case_test.rb

@@ -211,6 +211,15 @@ class AssertDifferenceTest < ActiveSupport::TestCase
     end
   end
 
+  def test_assert_changes_with_message
+    error = assert_raises Minitest::Assertion do
+      assert_changes "@object.num", "@object.num should 1", to: 1 do
+      end
+    end
+
+    assert_equal "@object.num should 1.\n\"@object.num\" didn't change to 1", error.message
+  end
+
   def test_assert_no_changes_pass
     assert_no_changes "@object.num" do
       # ...