Implicit Promise.all() resolves promise too eagerly.

[AlexMax]

Thanks to spion from Node.js IRC for helping me pinpoint a test case:

P.resolve(1).then(function(x) {
  return [x, P.resolve(2)]
}).spread(function(x, y) {
  return P.all([P.resolve(3), P.resolve(4)]);
}).then(function(a) {
  console.log(a);
});

// Expected: [3, 4]
// Actual: [1, 2]

Adding an explicit Promise.all() to the first return produces expected results.