Allow body to be consumed only once

[dgraham]

This branch enforces Section 5.2 of the spec, which describes how the bodyUsed attribute restricts consuming the body more than once.

fetch('/users').then(function(response) {
  // This promise resolves with the json object.
  var works = response.json()
  // This promise now rejects with a TypeError.
  var fails = response.text()
})

/cc @annevk on spec compliance here

[jonathanong]

you should always return a promise. return error ? Promise.reject(err) : Promise.resolve(...)

[jonathanong]

jk i didn't look up. hahaha

[dgraham]

Yeah, the name probably wasn't great. I just changed it from error to rejected to match expectations about its type.

[dgraham]

@annevk I'm working on a branch where the body is resolved later than the initial response promise. Right now, it's resolving the response in an XHR onload handler, so it has the full body as well.