Remove IsPromise check from GetDeferred

[domenic]

This is related to #51 and proposes a different fix for it than was recently implemented in e2391248b0bd2c485b9f95c56c96dc5b8d0cf5e4.

Proposal

• Remove the IsPromise check from GetDeferred
• Either:
  • Fix Then/Promise.prototype.then to use a different mechanism, instead of GetDeferred, for constructing its return value. (easy)
  • Pass the deferred.[[Resolve]] and deferred.[[Reject]] functions through the algorithms so that they can be used to resolve/reject the return value. (hard)
  • I am not sure what the differences would be.

    Observable Differences

• This will allow non-subclass constructors to be used as the this of static promise methods.

• In particular, the following will work:

  // Will now return a RSVP.Promise instance.
  Promise.all.call(RSVP.Promise, [...args]);
  
  // The polyfilling use case:
  RSVP.Promise.all = Promise.all || (iterable) => { ... };

• This will not impact Promise.prototype.then, e.g. the following will not work:

  // DOES NOT WORK
  Promise.prototype.then.call(rsvpPromiseInstance, f, r);
  RSVP.Promise.prototype.then = Promise.prototype.then || (f, r) => { ... };

  The IsPromise check inside Then/Promise.prototype.then will stay, since the whole point of that algorithm is to send internal state out via the onFulfilled and onRejected arguments.