Specification is incomplete re. payloads in request body

[fgabolde]

The specification does not handle at all parameters passed via the
request body, for e.g. POST, PATCH or PUT.

The notion of payload is briefly mentioned in the specification but
does not reappear in the list of parameters available for e.g. method
description.

Using the Net::HTTP::Spore module, a method spec like

"login" : {
   "path" : "/blah/:format",
   "method" : "POST",
   "description" : "Creates a new session for the current client.",
   "required_params" : [
      "format",
      "username",
      "password"
   ]
},

results in a POST request with an empty body and both the username and
password method passed through the query string. This agrees with the
spec as published, but is not the intended result.

The spore_validation.rx file has three "payload", "optional_payload"
and "required_payload" parameters but they are not documented
anywhere.