Drop MUST requirement for rel=self

[cweiske]

WebSub requires publishers to add a rel=self link to each document:

The discovery mechanism aims at identifying at least 2 URLs.
...
The canonical URL for the topic to which subscribers are expected to use for subscriptions.
The protocol currently supports the following discovery mechanisms. Publishers MUST implement at least one of them:
...

Please drop the MUST requirement for the rel=self URL and let clients fall back to URL that was used to retrieve the document. This will make it easier to implement WebSub on the publisher side.

Also see the discussion in the #indieweb channel: https://chat.indieweb.org/2015-05-22#t1432330396549000

[voxpelli]

Do someone know the historical reason it was added in the first place?

[pfefferle]

here is the original discussion: pubsubhubbub/PubSubHubbub#36

[cweiske]

I already forgot about that issue :/

[pfefferle]

rel-self is defined here btw. https://tools.ietf.org/html/rfc5988#section-6.2.2