use `inspect` for show `from` value

If `from` is nil, in order to avoid the blank is showed.

activesupport/lib/active_support/testing/assertions.rb

@@ -147,7 +147,7 @@ module ActiveSupport
         retval = yield
 
         unless from == UNTRACKED
-          error = "#{expression.inspect} isn't #{from}"
+          error = "#{expression.inspect} isn't #{from.inspect}"
           error = "#{message}.\n#{error}" if message
           assert from === before, error
         end

activesupport/test/test_case_test.rb

@@ -138,6 +138,15 @@ class AssertDifferenceTest < ActiveSupport::TestCase
     end
   end
 
+  def test_assert_changes_with_from_option_with_nil
+    error = assert_raises Minitest::Assertion do
+      assert_changes "@object.num", from: nil do
+        @object.increment
+      end
+    end
+    assert_equal "\"@object.num\" isn't nil", error.message
+  end
+
   def test_assert_changes_with_to_option
     assert_changes "@object.num", to: 1 do
       @object.increment