Once maxSize is reached calculations don't account for duplicates

[lukechilds]

Because there are two Maps when you overflow maxSize you can't rely on all of the items to be unique.

This means lru.size is incorrect, it will count duplicates when it should only count each key once.

This also means the iterables are incorrect.

e.g:

const lru = new QuickLRU({ maxSize: 2 });
lru.set('key', 'value');
lru.set('shouldOnlyAppearOnce', 1);
lru.set('shouldOnlyAppearOnce', 2);

lru.size;
// 3

for(keyValue of lru) {
  console.log(keyValue);
}
// [ 'shouldOnlyAppearOnce', 2 ]
// [ 'key', 'value' ]
// [ 'shouldOnlyAppearOnce', 1 ]

Same issue affects generator functions for lru.keys and lru.values.

[lukechilds]

Should probably clarify, all other methods are unaffected.