notifier: ensure Content-Type header present in webhook notification

[alecmerdler]

When receiving the notification webhook in Quay, we call Flask's .json() method. This returns None if the Content-Type header is not set to application/json.

[hdonnay]

I think this would be better written as:

Suggested change

	var headers http.Header = map[string][]string{}
	headers["Content-Type"] = []string{"application/json"}
	for key, val := range d.conf.Headers {
	headers[key] = val
	}
	headers := d.conf.Headers.Clone()
	headers.Set("Content-Type", "application/json")

[alecmerdler]

What if Headers is nil?

[hdonnay]

Hm yeah, that eats it because Clone returns nil. We could make Validate ensure it's non-nil, or not do it my suggested way. @ldelossa thoughts?

[ldelossa]

I think doing this in Validate method makes sense.

The webhook deliverer will always be sending json data. So in the "Validate()" method, merge any incoming headers with the "Content-type" header and construct the deliverer with the the merged http.Header map.