Merge branch '6.4' into 7.1

* 6.4:
  Remove comment about AppVeyor in `phpunit`
  [Webhook][RemoteEvent] fix SendgridPayloadConverter category support
  Update old Appveyor skip conditions
  sync the Dutch translation file with changes from the 7.2 branch
  [Yaml] fix inline notation with inline comment
  fix(property-info): make sure that SerializerExtractor returns null for invalid class metadata
  [RemoteEvent][Webhook] fix SendgridRequestParser & SendgridPayloadConverter in case of missing sg_message_id
  fix_50486 - memory leak

Author: xabbuh

Tests/Dumper/ServerDumperTest.php

@@ -39,8 +39,8 @@ public function testDumpForwardsToWrappedDumperWhenServerIsUnavailable()
 
     public function testDump()
     {
-        if ('True' === getenv('APPVEYOR')) {
-            $this->markTestSkipped('Skip transient test on AppVeyor');
+        if ('\\' === \DIRECTORY_SEPARATOR) {
+            $this->markTestSkipped('Skip transient test on Windows');
         }
 
         $wrappedDumper = $this->createMock(DataDumperInterface::class);

Tests/Server/ConnectionTest.php

@@ -24,8 +24,8 @@ class ConnectionTest extends TestCase
 
     public function testDump()
     {
-        if ('True' === getenv('APPVEYOR')) {
-            $this->markTestSkipped('Skip transient test on AppVeyor');
+        if ('\\' === \DIRECTORY_SEPARATOR) {
+            $this->markTestSkipped('Skip transient test on Windows');
         }
 
         $cloner = new VarCloner();