Add disclaimer for optimistic lock unexisting key

aiocache/lock.py

@@ -136,6 +136,8 @@ class OptimisticLock:
             result = await super_expensive_call()
             await cache.set('random_value')  # This will make the `lock.cas` call fail
             await lock.cas(result)
+
+    If the lock is created with an unexisting key, there will never be conflicts.
     """
 
     def __init__(self, client: Type[BaseCache], key: str):

tests/acceptance/test_lock.py

@@ -167,8 +167,9 @@ async def test_release_does_nothing(self, lock):
         assert await lock.__aexit__("exc_type", "exc_value", "traceback") is None
 
     @pytest.mark.asyncio
-    async def test_check_and_set_not_existing(self, cache, lock):
+    async def test_check_and_set_not_existing_never_fails(self, cache, lock):
         async with lock as locked:
+            await cache.set('conflicting_value')
             await locked.cas('value')
 
         assert await cache.get(pytest.KEY) == 'value'