forEach iterates over all entries when an entry is deleted

[timjk]

forEach now continues if an item is deleted from the cache whilst iterating over it, fixing issue #33.

A consequence of this change is if one adds an entry during the forEach it will not be iterated over. If this behaviour is wanted a check could be added sometime after the fn.call(...) inside the for loop e.g.

if (this._itemCount > itemCount) {
  itemCount = this._itemCount
}

[isaacs]

Thanks, landed on master.