`sender` changes in the `check_run` webhook depending on the committer

[travi]

this highlights my misunderstanding of what sender is intended to mean. i guess it should mean the user that triggered the event, not the user that opened the PR. makes sense, but i went down the wrong path for some reason.

user should, instead, be pulled from the pull_request resource, like is done for the status event. however, that detail is not included in the check_run payload, so the complete version of the pull_request will need to be fetched first.