Include amount changed by in `assert_difference` failure message

Unless you're very good at math, this test fail message is not the easiest to debug:

```
"User.count" didn't change by 32.
Expected: 1611
  Actual: 1579
```

It's not obvious from the error, but in this case, it actually changed by 0. This is a pretty strong clue as to what went wrong, but the message doesn't tell us that.

This PR improves the message to make debugging easier:

```
"User.count" didn't change by 32 (changed by 0).
Expected: 1611
  Actual: 1579
```

activesupport/CHANGELOG.md

@@ -1,3 +1,25 @@
+*   `assert_difference` message now includes what changed.
+
+    This makes it easier to debug non-obvious failures.
+
+    Before:
+
+    ```
+    "User.count" didn't change by 32.
+    Expected: 1611
+      Actual: 1579
+    ```
+
+    After:
+
+    ```
+    "User.count" didn't change by 32, but by 0.
+    Expected: 1611
+      Actual: 1579
+    ```
+
+    *Alex Ghiculescu*
+
 *   Add ability to match exception messages to `assert_raises` assertion
 
     Instead of this

activesupport/lib/active_support/testing/assertions.rb

@@ -115,9 +115,10 @@ module ActiveSupport
         retval = _assert_nothing_raised_or_warn("assert_difference", &block)
 
         expressions.zip(exps, before) do |(code, diff), exp, before_value|
-          error  = "#{code.inspect} didn't change by #{diff}"
+          actual = exp.call
+          error  = "#{code.inspect} didn't change by #{diff}, but by #{actual - before_value}"
           error  = "#{message}.\n#{error}" if message
-          assert_equal(before_value + diff, exp.call, error)
+          assert_equal(before_value + diff, actual, error)
         end
 
         retval

activesupport/test/test_case_test.rb

@@ -54,7 +54,7 @@ class AssertionsTest < ActiveSupport::TestCase
         @object.increment
       end
     end
-    assert_equal "\"@object.num\" didn't change by 0.\nExpected: 0\n  Actual: 1", error.message
+    assert_equal "\"@object.num\" didn't change by 0, but by 1.\nExpected: 0\n  Actual: 1", error.message
   end
 
   def test_assert_no_difference_with_message_fail
@@ -63,7 +63,7 @@ class AssertionsTest < ActiveSupport::TestCase
         @object.increment
       end
     end
-    assert_equal "Object Changed.\n\"@object.num\" didn't change by 0.\nExpected: 0\n  Actual: 1", error.message
+    assert_equal "Object Changed.\n\"@object.num\" didn't change by 0, but by 1.\nExpected: 0\n  Actual: 1", error.message
   end
 
   def test_assert_no_difference_with_multiple_expressions_pass
@@ -157,7 +157,17 @@ class AssertionsTest < ActiveSupport::TestCase
         @object.increment
       end
     end
-    assert_equal "Object Changed.\n\"@object.num\" didn't change by 0.\nExpected: 0\n  Actual: 1", error.message
+    assert_equal "Object Changed.\n\"@object.num\" didn't change by 0, but by 1.\nExpected: 0\n  Actual: 1", error.message
+  end
+
+  def test_assert_difference_message_includes_change
+    error = assert_raises Minitest::Assertion do
+      assert_difference "@object.num", +5 do
+        @object.increment
+        @object.increment
+      end
+    end
+    assert_equal "\"@object.num\" didn't change by 5, but by 2.\nExpected: 5\n  Actual: 2", error.message
   end
 
   def test_hash_of_lambda_expressions