Return type of grabHttpHeader should be string|array|null

[iweindesmedt]

... so that it matches the return type of ...->getInternalResponse()->getHeader()

I'm not sure where the ActorActions are generated, but those generated methods should also match this return type.

https://github.com/Codeception/module-rest/blob/master/src/Codeception/Module/REST.php#L313

[Naktibalda]

Released as 3.2.0 https://github.com/Codeception/module-rest/releases/tag/3.2.0