Fix cache size becoming double of the defined size (sindresorhus#13)

index.js

@@ -30,6 +30,7 @@ class QuickLRU {
 
 		if (this.oldCache.has(key)) {
 			const value = this.oldCache.get(key);
+			this.oldCache.delete(key);
 			this._set(key, value);
 			return value;
 		}

test.js

@@ -166,3 +166,11 @@ test('.size - accounts for dupes', t => {
 	const lru = lruWithDuplicates();
 	t.is(lru.size, 2);
 });
+
+test('checks total cache size does not exceed maxSize', t => {
+	const lru = new QuickLRU({maxSize: 2});
+	lru.set('1', 1);
+	lru.set('2', 2);
+	lru.get('1');
+	t.is(lru.oldCache.has('1'), false);
+});