Test Alert will not be shown in AlertManager

[StefanHufschmidt]

After pressing on the Test button for the callback in Graylog you'll get a successful-notification in Graylog but you will not get any alert in your AlertManager.

With an invalid configuration the test will fail. So there must be something wrong transmitted to the AlertManager API.

I took a look into it and could not find the root cause of it. Even after catching every possible nullpointer exception and setting everything which is required for the POST-request to the AlertManager API (see https://github.com/prometheus/docs/blob/master/content/docs/alerting/clients.md) the problem still exists.

Currently the only way you can test your configuration properly is to define a test-condition which will trigger the alert.

[StefanHufschmidt]

While working on #11 I've noticed that the test alert works after the related changes when using AlertManager version 0.20.0. I think this issue will be solved by the related changes of #11 .