'Adds a new key value pair to the cache.
Parameters
key : any
The key to use for the pair.
value : any
The value to store under the name ``key``.
expiration_dt : datetime
When should this mapping expire? The cache is considered invalid
for dates **strictly greater** than ``expiration_dt``.'

def set(self, key, value, expiration_dt):

    self._cache[key] = CachedObject(value, expiration_dt)
