Consider supporting application/x-www-form-urlencoded as content type for webhook

[ocean90]

Issue Overview
When using application/x-www-form-urlencoded and not application/json as the content type for the incoming webhook you get the message {"result":"Not the default branch"}.
I'd expect another message if the content type isn't supported at all or, maybe be better, actually supported.

To Reproduce
Steps to reproduce the behavior:

1. Go to a GitHub project
2. Change the content type of the webhook
3. Scroll down to a previous delivery
4. Click Redeliver and check the response tab

Expected behavior
An better error message or a successful import.