Providing a subject key in a structured payload falls into the extension flow. It is then rejected as being a reserved attribute name.

I did not open a PR yet, I wanted to ensure this was not intended behavior. Failing test shown below.

https://github.com/cloudevents/sdk-javascript/compare/master...jabrown85:jab/subject-unmarshaller?expand=1

227 passing (226ms)
  1 failing

  1) HTTP Transport Binding Unmarshaller for CloudEvents v0.3
       Structured
         Should accept event that follow the spec 0.3:
     Reserved attribute name: 'subject'