Memoizing None

[unformatt]

I see this PR which adds support for memoizing None #17 but it seems that some refactoring later might have broken it?

>>> python ./manage.py shell

from memoize import memoize

@memoize()
def a(x):
    print 'a not memoized'
    if x == 1:
        return None
    return 'foo'

a(2)
a(2)
a(1)
a(1)

Seems to be cause by cache.get specifying a default if None is the value:

def get(self, key):
        "Proxy function for internal cache object."
        return self.cache.get(key=key, default=self.default_cache_value)