Fixing nil http.Result ack compare

[n3wscott]

fixes #558

I found two issues in the codebase:

1. the client was using the old style !IsACK to understand if it should process the response into a CloudEvent. The thinking is if it was not an ACK, then there must not be any event to parse. This logic is incorrect, it should test for IsUndelivered. If the event never left, then it can't have a response.
2. Nil http protocol Results are not treated as ACK.

Running the samples/http/responder and samples/http/requester now get the expected output:

2020/07/27 14:16:18 Event sent at 2020-07-27 14:16:18.236459 -0700 PDT m=+3.540167810
2020/07/27 14:16:18 Response status code 500
Response:
Validation: valid
Context Attributes,
  specversion: 1.0
  type: samples.http.mod7
  source: /mod7
  subject: https://github.com/cloudevents/sdk-go/v2/samples/requester#7
  id: 10a6563c-6400-4782-8654-39180bf8f080
  time: 2020-07-27T21:16:18.236026Z
  datacontenttype: application/json
Data,
  {
    "id": 7,
    "message": "mod 7 has issues!"
  }

Signed-off-by: Scott Nichols snichols@vmware.com

[slinkydeveloper]

Just a little change to the example, but lgtm

[slinkydeveloper]

Can you add a little comment here?

If event is delivered then the result can be parsed as http result

And later

response could contain or not an event