"assigned" Action on `PullRequestEvent` returns 400

[baoist]

Hey,

I've been testing PullRequestEvents with an action of "assigned" and it returns with a 400 status code.

Status=400 X-GitHub-Event="pull_request" Content-Length=20381: json: cannot unmarshal object into Go value of type string

Also, I believe if there is an assignee, all subsequent requests fail. I will look into this a bit more later unless you know of a quick/obvious fix.