Stream creation lanaguage imply that transmiter must suport the poll delivery method if delivery method is not specified

[ysarig1]

Section 7.1.1.1 (stream creation) have the following language: "If the request does not contain the delivery property, then the Transmitter MUST assume that the method is "urn:ietf:rfc:8936" (poll). The Transmitter MUST include a delivery property in the response with this method property and an endpoint_url property."

The language seem to imply that the transmitter must support the poll method, which shouldn't be the case as supporting the poll method is optional.

The language should be changed that a 400 response should be returned if the poll method is not supported when delivery is not specified in the request.

[tulshi]

This is a backward-incompatible change, so we should not include this in v3.