Add slow request GitHub issue middleware

Open a GitHub issue when a request takes too much time. If a slow request
to the same URL occurs more than once the middleware updates the associated
issue by reopening it.
Merge branch 'slow-request-github-issue' of https://github.com/potomak/middleware into potomak-slow-request-github-issue
send json body to github
Merge branch 'potomak-slow-request-github-issue'