Merge 3.9.x 201908071359 to master (#6484)

* Add changelog for 3.9.x

* Revert "Make php message class final to avoid mocking (#6277)" (#6324)

This reverts commit 7f84a943667358a53bd5d255ee9769ca83e75cd9.
This is just temporary. Eventually, we still want to roll forward this
change. Some users are complaining they need more time to clean up their
code.

* Update extract_includes.bat.in

File io_win32.h is not in directory google\protobuf\stubs under directory google\protobuf\io

* Set oneof case in array constructor (#6351)

Forgot to set it previously.

* Update protobuf version (#6366)

* Drop building wheel for python 3.4 (#6406)

https://github.com/matthew-brett/multibuild/pull/240

* Fix binary compatibility in FieldCodec factory methods (#6380) (#6424)

* Fix binary compatibility in FieldCodec factory messages

* Make default value parameter for current factories required

* Route old methods through default value overloads

* Remove ExtensionRegistry.Add(params) overload

* Rename ExtensionRegistry.Add(IEnumerable<Extension>) overload to AddRange

* Edit naming of parameters in Extension classes

* * Fix add API warnings to docs for extension APIs
* Rename internal ExtensionSet.GetValue to TryGetValue

* Disable javadoc error (#6371)

* Disable javadoc error

Actual fixes of the javadoc will be followed up

* Remove duplicated configuration

* Update javadoc plugin version

* Updated Bazel test script to use most recent Bazel version (#6413) (#6433)

I'm not exactly sure why, but this fixes the failing Bazel presubmit
test. Using the most recent version seems like a good idea anyway so
that we can make sure we're compatible with any new Bazel changes.

* [bazel] Add fixes for --incompatible_load_{cc,java,proto}_rules_from_bzl

* No need to update version in generated code (#6471)

generate_descriptor will handle that

* Update protobuf version (#6472)
diff --git a/php/tests/generated_class_test.php b/php/tests/generated_class_test.php
index a54c4e5..6af00ef 100644
--- a/php/tests/generated_class_test.php
+++ b/php/tests/generated_class_test.php
@@ -1452,6 +1452,8 @@
         $m = new TestMessage([
             'oneof_message' => new Sub(),
         ]);
+        $this->assertSame('oneof_message', $m->getMyOneof());
+        $this->assertNotNull($m->getOneofMessage());
     }
 
     public function testOneofStringInArrayConstructor()