cache argument to lru_cache_function isn't supported

If you try this code from the docs:

```
d = LRUCacheDict(max_size=3, expiration=3, thread_clear=True)

@lru_cache_function(cache=d)
def f(x):
    return x/2

```

You'll get this error:
`TypeError: lru_cache_function() got an unexpected keyword argument 'cache'`
