Populate 'created' property on ostatus_source

The 'created' column in ostatus_source SQL table has NOT NULL restriction.
INSERTs fail when running MySQL/MariaDB in strict mode if this is not
populated.

plugins/OStatus/classes/Ostatus_source.php

@@ -66,6 +66,7 @@ class Ostatus_source extends Managed_DataObject
         $osource->notice_id = $notice->id;
         $osource->profile_uri = $oprofile->uri;
         $osource->method = $method;
+        $osource->created = common_sql_now();
         if ($osource->insert()) {
            return true;
         } else {