GitHub IssueCrawler: PR merged or not

[robertmuehsig]

Invoke like this:
https://api.github.com/repos/aspnet/blazor/pulls/672/merge

and check for HTTP Response:
https://developer.github.com/v3/pulls/#get-if-a-pull-request-has-been-merged