Fix key-value formatting for json and csv result set

Remove parameter type for second argument (`$value`) in `CsvResultSet::formatValue` and `JsonResultSet::formatValue`.
Also add date time formatting for `$value` parameter in case it is an instance of `DateTime` object.

library/Icingadb/Data/CsvResultSet.php

@@ -4,6 +4,8 @@
 
 namespace Icinga\Module\Icingadb\Data;
 
+use DateTime;
+use DateTimeZone;
 use ipl\Orm\Model;
 use ipl\Orm\Query;
 use ipl\Orm\ResultSet;
@@ -17,7 +19,7 @@ public function current()
  return $this->extractKeysAndValues(parent::current());
  }
 
- protected function formatValue(string $key, ?string $value): ?string
+ protected function formatValue(string $key, $value): ?string
  {
  if (
  $value
@@ -38,6 +40,9 @@ protected function formatValue(string $key, ?string $value): ?string
  return '"' . str_replace('"', '""', $value) . '"';
  } elseif (is_array($value)) {
  return '"' . implode(',', $value) . '"';
+ } elseif ($value instanceof DateTime) {
+ return $value->setTimezone(new DateTimeZone('UTC'))
+ ->format('Y-m-d\TH:i:s.vP');
  } else {
  return $value;
  }

library/Icingadb/Data/JsonResultSet.php

@@ -4,6 +4,8 @@
 
 namespace Icinga\Module\Icingadb\Data;
 
+use DateTime;
+use DateTimeZone;
 use Icinga\Util\Json;
 use ipl\Orm\Model;
 use ipl\Orm\Query;
@@ -18,7 +20,7 @@ public function current()
  return $this->createObject(parent::current());
  }
 
- protected function formatValue(string $key, ?string $value): ?string
+ protected function formatValue(string $key, $value): ?string
  {
  if (
  $value
@@ -33,6 +35,11 @@ protected function formatValue(string $key, ?string $value): ?string
  $value = bin2hex($value);
  }
 
+ if ($value instanceof DateTime) {
+ return $value->setTimezone(new DateTimeZone('UTC'))
+ ->format('Y-m-d\TH:i:s.vP');
+ }
+
  return $value;
  }