Class.Thenable: Add getThenableState function.
Request/Fx: Only resetThenable when state is not pending.

Which should in practice mean it's always called except for the very first time, due to the `this.check` call making sure there is no running state anymore.
Class.Thenable: Basic specs (still TODO: integrate Promises/A+ tests).
Class.Thenable: Add getThenableState to docs.