Make assert_difference return the result of the yielded block.

With this we can perform new assertions on the returned value without having
to cache it with an outer variable or wrapping all subsequent assertions inside
the `assert_difference` block.

Before:

```
post = nil
assert_difference -> { Post.count }, 1 do
  Post.create
end

assert_predicate post, :persisted?
```

Now:

```
post = assert_difference -> { Post.count } do
  Post.create
end

assert_predicate post, :persisted?
```

activesupport/CHANGELOG.md

@@ -1,3 +1,14 @@
+*   `assert_difference` and `assert_no_difference` now returns the result of the
+    yielded block.
+
+    Example:
+
+      post = assert_difference -> { Post.count }, 1 do
+        Post.create
+      end
+
+    *Lucas Mazza*
+
 *   Short-circuit `blank?` on date and time values since they are never blank.
 
     Fixes #21657
@@ -12,7 +23,7 @@
 *   Updated Unicode version to 8.0.0
 
     *Anshul Sharma*
-    
+
 *   `number_to_currency` and `number_with_delimiter` now accept custom `delimiter_pattern` option
      to handle placement of delimiter, to support currency formats like INR
 

activesupport/lib/active_support/testing/assertions.rb

@@ -68,13 +68,15 @@ def assert_difference(expression, difference = 1, message = nil, &block)
         }
         before = exps.map(&:call)
 
-        yield
+        retval = yield
 
         expressions.zip(exps).each_with_index do |(code, e), i|
           error  = "#{code.inspect} didn't change by #{difference}"
           error  = "#{message}.\n#{error}" if message
           assert_equal(before[i] + difference, e.call, error)
         end
+
+        retval
       end
 
       # Assertion that the numeric result of evaluating an expression is not

activesupport/test/test_case_test.rb

@@ -56,6 +56,14 @@ def test_assert_difference
     end
   end
 
+  def test_assert_difference_retval
+    incremented = assert_difference '@object.num', +1 do
+      @object.increment
+    end
+
+    assert_equal incremented, 1
+  end
+
   def test_assert_difference_with_implicit_difference
     assert_difference '@object.num' do
       @object.increment