adds a test for the protocol dumping of a request to the containert stdin.
there are a number of vectors to test for a cloud event, but since we're going
to change that behavior soon it's probably a waste of time to go about doing
so. in any event, this was pretty broken. my understanding of the cloud event
spec is deepening and the json stuff overall seems a little weird.
* fixes content type issue around json checking (since a string is also a json
value, we can just decode it, even though it's wasteful it's more easily
correct)
* doesn't force all json values to be map[string]interface{} and lets them be
whoever they want to be. maybe their dads are still proud.
closes#966
