Webhook GET bugs

[techknowlogick]

#6771 is missing a few things:

• Logs sent as POST when set to GET even though it is indeed set to GET (test found here: https://try.gitea.io/techknowlogick/test/settings/hooks/199 )
• Can't change from GET to POST (or vice versa)

Perhaps unrelated, but there have been scattered reports of webhooks being unable to send and setting date as 0 unix time. See https://gitea.com/techknowlogick/test-repo/settings/hooks/18 for example