Bring back assert_serializer for controller testing

The `assert_serializer` test helper was added in 0.9.0.apha1[1],
and was not included in 0.10.

This patch brings back the `assert_serializer` test helper. This is the last
revision[2] that has the helper. The original helper was used as base.

[1]: https://github.com/rails-api/active_model_serializers/pull/596
[2]: 610aeb2e92

- Create the AssertSerializer

- Use the Test namespace

- Make the tests pass on the Rails master
  - Rails 5 does not include `assert_template` but we need this on the tests of
the helper.
  - This add the `rails-controller-testing` to keep support on `assert_template`.

- Only load test helpers in the test environment

docs/README.md

@@ -22,6 +22,7 @@ This is the documentation of ActiveModelSerializers, it's focused on the **0.10.
 - [How to add root key](howto/add_root_key.md)
 - [How to add pagination links](howto/add_pagination_links.md)
 - [Using ActiveModelSerializers Outside Of Controllers](howto/outside_controller_use.md)
+- [Testing ActiveModelSerializers](howto/test.md)
 
 ## Integrations
 

docs/howto/test.md

@@ -0,0 +1,29 @@
+# How to test
+
+## Test helpers
+
+ActiveModelSerializers provides a `assert_serializer` method to be used on your controller tests to
+assert that a specific serializer was used.
+
+```ruby
+class PostsControllerTest < ActionController::TestCase
+  test "should render post serializer" do
+    get :index
+    assert_serializer "PostSerializer"
+    #  # return a custom error message
+    #  assert_serializer "PostSerializer", "PostSerializer not rendered"
+    #
+    #  # assert that the instance of PostSerializer was rendered
+    #  assert_serializer PostSerializer
+    #
+    #  # assert that the "PostSerializer" serializer was rendered
+    #  assert_serializer :post_serializer
+    #
+    #  # assert that the rendered serializer starts with "Post"
+    #  assert_serializer %r{\APost.+\Z}
+    #
+    #  # assert that no serializer was rendered
+    #  assert_serializer nil
+  end
+end
+```