Fix issue with duedate format

Signed-off-by: Joas Schilling <coding@schilljs.com>

lib/Db/RelationalEntity.php

@@ -72,6 +72,9 @@ class RelationalEntity extends Entity implements \JsonSerializable {
 				$propertyReflection = $reflection->getProperty($property);
 				if (!$propertyReflection->isPrivate() && !in_array($property, $this->_resolvedProperties, true)) {
 					$json[$property] = $this->getter($property);
+					if ($json[$property] instanceof \DateTimeInterface) {
+						$json[$property] = $json[$property]->format('c');
+					}
 				}
 			}
 		}