Make assertEquals pass for message (#4947)

This change only makes assertEquals pass for message in c extension. However, it actually does nothing.
This is the same behavior before 3.6.0 release.
diff --git a/php/tests/generated_class_test.php b/php/tests/generated_class_test.php
index 9f20bdd..8bac4e5 100644
--- a/php/tests/generated_class_test.php
+++ b/php/tests/generated_class_test.php
@@ -1342,4 +1342,17 @@
 
         TestUtil::assertTestMessage($m);
     }
+
+    #########################################################
+    # Test message equals.
+    #########################################################
+
+    public function testMessageEquals()
+    {
+        $m = new TestMessage();
+        TestUtil::setTestMessage($m);
+        $n = new TestMessage();
+        TestUtil::setTestMessage($n);
+        $this->assertEquals($m, $n);
+    }
 }