Fix bug in Cache.set(): self.map[query] is not defined in constructor, replaced with self.lookup.get(query)

[WillyPhan06]

Previous Code

class Cache(object):

    def __init__(self, MAX_SIZE):
        self.MAX_SIZE = MAX_SIZE
        self.size = 0
        self.lookup = {}
        self.linked_list = LinkedList()

    def set(self, results, query):
        node = self.map[query]        

In the set function:
self.map[query] was used to retrieve query but it is not defined in the constructor.
So I replaced it with self.lookup.get(query) to correctly do that based on the constructor.