Allow for custom iTipBroker (sabre-io#1559)

Signed-off-by: SebastianKrupinski <krupinskis05@gmail.com>

lib/CalDAV/Schedule/Plugin.php

@@ -25,6 +25,7 @@
 use Sabre\VObject;
 use Sabre\VObject\Component\VCalendar;
 use Sabre\VObject\ITip;
+use Sabre\VObject\ITip\Broker;
 use Sabre\VObject\ITip\Message;
 use Sabre\VObject\Reader;
 
@@ -389,7 +390,7 @@ public function beforeUnbind($path)
             $node->getOwner()
         );
 
-        $broker = new ITip\Broker();
+        $broker = $this->createITipBroker();
         $messages = $broker->parseEvent(null, $addresses, $node->get());
 
         foreach ($messages as $message) {
@@ -500,7 +501,7 @@ public function scheduleLocalDelivery(ITip\Message $iTipMessage)
             $isNewNode = true;
         }
 
-        $broker = new ITip\Broker();
+        $broker = $this->createITipBroker();
         $newObject = $broker->processMessage($iTipMessage, $currentObject);
 
         $inbox->createFile($newFileName, $iTipMessage->message->serialize());
@@ -611,7 +612,7 @@ public function getSupportedPrivilegeSet(INode $node, array &$supportedPrivilege
      */
     protected function processICalendarChange($oldObject, VCalendar $newObject, array $addresses, array $ignore = [], &$modified = false)
     {
-        $broker = new ITip\Broker();
+        $broker = $this->createITipBroker();
         $messages = $broker->parseEvent($newObject, $addresses, $oldObject);
 
         if ($messages) {
@@ -994,4 +995,12 @@ public function getPluginInfo()
             'link' => 'http://sabre.io/dav/scheduling/',
         ];
     }
+
+    /**
+     * Returns an instance of the iTip\Broker.
+     */
+    protected function createITipBroker(): Broker
+    {
+        return new Broker();
+    }
 }