Fix/unable to use custom params

[ishiko732]

code:

const fsrs = new FSRS([
  0.4911, 4.5674, 24.8836, 77.045, 7.5474, 0.1873, 1.7732, 0.001, 1.1112,
  0.152, 0.5728, 1.8747, 0.1733, 0.2449, 2.2905, 0.0, 2.9898, 0.0883,
  0.9033,
]);

const desiredRetention = 0.9;
const t = 0
const memory = fsrs.nextStates(null, desiredRetention, t);
console.log('again', +memory.again.memory.stability.toFixed(2), +memory.again.memory.difficulty.toFixed(2));
console.log('hard', +memory.hard.memory.stability.toFixed(2), +memory.hard.memory.difficulty.toFixed(2));
console.log('good', +memory.good.memory.stability.toFixed(2), +memory.good.memory.difficulty.toFixed(2));
console.log('easy', +memory.easy.memory.stability.toFixed(2), +memory.easy.memory.difficulty.toFixed(2));

before:

again 0.4 7.19
hard 1.18 6.49
good 3.17 5.28
easy 15.69 3.22

after:

again 0.49 7.55
hard 4.57 7.34
good 24.88 7.09
easy 77.04 6.79

[L-M-Sherlock]

LGTM