HTTP Send results in IsUndelivered true for non-2XX responses

[sbgreene1307]

Expected Behaviour

When invoking cloudeventshttp.Send non-2XX responses should come back as a ResultNACK.

In v2/protocol/http/protocol_retry.go all non-2XX responses result in a ResultNACK which should come back as the return from Send.

Actual Behaviour

In v2/protocol/http/protocol.go the ResultNACK is lost due to creating a new http result with a new error string rather than the ResultNACK as a wrapped error.

This results in IsUndelivered being true.

[n3wscott]

Would you be able to repo this with a test?

[sbgreene1307]

Sure, I thought the integration test dir would be a good spot for this. I had a little trouble tying into the current implementation to trigger a different response code, if you know of how it can be done with the TapHandler I'm all ears.

This at least demonstrates the issue, I can rework anything as necessary.
#763