Bug in default

In these method, and probably others, the `default` parameter is not sent to the underlying dict: None is always sent.

https://github.com/deterministic-arts/DartsPyLRU/blob/master/darts/lib/utils/lru.py#L552-L584

This breaks the functionality. Example PoC:

```
lru = SynchronizedLRUDict()
default = lru.get('foo', default=1)
assert default==1
```