Add updatePullRequest method

To update a pull request via the github API you need to supply the same options as you did during creation, but should use `PATCH` instead of `POST`.

The other `PATCH` functions I saw weren't consistent about names (`editXXX` vs `updateXXX`) and whether it was required to pass in an ID of the thing you're modifying. I went with `updatePullRequest` because I think that's the term github uses. I also opted to not pass in the PR ID because it's not strictly necessary, assuming you're using the same base/head params.