`GET /repos/{owner}/{repo}/pulls` is documented with `state` defaulting
to `open`, but without `state` it returned every pull request, closed
and merged ones included. Now it parses `state` with
`ParseIssueFilterStateIsClosed`, like the issues and milestones
endpoints, so no state means open, as documented. `closed` and `all`
work as before. GitHub's pulls endpoint also defaults to open.