gh: data races in tests

[rjeczalik]

WARNING: DATA RACE
Read at 0x00c4200ac108 by goroutine 12:
  net/http.(*response).finishRequest()
      /home/travis/.gimme/versions/go1.8.3.linux.amd64/src/net/http/server.go:1534 +0x57
  net/http.(*conn).serve()
      /home/travis/.gimme/versions/go1.8.3.linux.amd64/src/net/http/server.go:1830 +0x76f
Previous write at 0x00c4200ac108 by goroutine 16:
  net/http.(*response).WriteHeader()
      /home/travis/.gimme/versions/go1.8.3.linux.amd64/src/net/http/server.go:1056 +0xb9
  github.com/rjeczalik/gh/webhook.(*Handler).handle()
      /home/travis/gopath/src/github.com/rjeczalik/gh/webhook/handler.go:234 +0x8f3

https://travis-ci.org/rjeczalik/gh/builds/256835732