PubSubHubbub notification is missing content-type header

[aaronpk]

The PubSubHubbub spec (and the new WebSub spec) both specify that the payload must contain a content-type header that matches the topic URL.

I believe this should be a simple fix just adding the content type header here.

You can test this using the WebSub test suite at https://websub.rocks/hub/100

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• I searched or browsed the repo’s other issues to ensure this is not a duplicate.
• This bug happens on a tagged release and not on master (If you're a user, don't worry about this).