handle GET requests in webhook hra

[robwhitby]

Hi,

The logs of the webhook controller show an error every 30secs, I guess due to a health check request which fails to be parsed as a GitHub event.

2021-03-08T14:41:55.562Z	ERROR	controllers.Runner	error validating request body	{"error": "Webhook request has unsupported Content-Type \"\""}
2021-03-08T14:42:25.556Z	ERROR	controllers.Runner	error validating request body	{"error": "Webhook request has unsupported Content-Type \"\""}
2021-03-08T14:42:55.558Z	ERROR	controllers.Runner	error validating request body	{"error": "Webhook request has unsupported Content-Type \"\""}

Currently the handler assumes all requests are POSTs from GitHub, this PR changes it to return early with 200 OK for any GET request.

[mumoshu]

Awesome! Thanks a lot for your contribution @robwhitby ☺️