Consider testing against Promise A+ spec's conformation tests

[kevincfz]

This is an awesome great functional implementation that uses state machine!
Many other implementations use too much repeated if checks.

It would greater if Nancy can pass all the Promise A+ spec tests: https://github.com/promises-aplus/promises-tests

One place where Nancy breaks is when empty then are chained. e.g.

const a = new Nancy((resolve, reject) => {
    setTimeout(() => {
      resolve('123');
    }, 1000);
  })
    .then()
    .then((value) => {
      console.log(value);
    });