No support for maintainer_can_modify parameter to create_pull

[mdbooth]

I have an app which creates a pull request from a repo it doesn't have write access to. This results in a 422 response from GitHub with the message: "fork_collab Fork collab can't be granted by someone without permission". The reason for this is that by default maintainer_can_modify is true, but without write permission on the source repo the app can't grant modify permission to the maintainers of the destination repo.

Repo.create_pull() doesn't allow me to set maintainer_can_modify, so I'm not able to use it.

Incidentally, I have worked round it with a call to _post(), so I can confirm that setting maintainer_can_modify to false fixes my issue.