When cache contains 2 items updating the first creates a reference loop in the second

[avoidwork]

Noticed while fixing #4

> const cache = require("./lib/tiny-lru.es6")(10);
undefined
> cache.set('1', 'a');
LRU {
  cache: { '1': { next: null, previous: null, value: 'a' } },
  first: '1',
  last: '1',
  length: 1,
  max: 10,
  notify: false,
  onchange: [Function],
  update: [Function] }
> cache.set('2', 'b');
LRU {
  cache: 
   { '1': { next: '2', previous: null, value: 'a' },
     '2': { next: null, previous: '1', value: 'b' } },
  first: '2',
  last: '1',
  length: 2,
  max: 10,
  notify: false,
  onchange: [Function],
  update: [Function] }
> cache.set('1', 'c');
LRU {
  cache: 
   { '1': { next: null, previous: '2', value: 'c' },
     '2': { next: '1', previous: '1', value: 'b' } },
  first: '1',
  last: '2',
  length: 2,
  max: 10,
  notify: false,
  onchange: [Function],
  update: [Function] }

The previous value of '2' should be null in this case, and not '1'