Fix the issue in FIFO retrieval in priorityQueueHeap.py

[amirsoroush]

Currently FIFO is not working correct.

In the following example "Ant" should come before "Lion":

>>> h = PriorityQueueHeap()
>>> h.insert(2, "Bat")
>>> h.insert(2, "Cat")
>>> h.insert(18, "Rat")
>>> h.insert(2, "Ant")
>>> h.insert(2, "Lion")
>>> h.insert(4, "Bear")
>>> 
>>> for i in range(h.size):
...     n = h.delete_at_root()
...     print(n)
... 
Bat
Cat
Lion
Ant
Bear
Rat
>>> 

The proposed change will fix this issue.

I also (just like #13) terminated the iteration as soon as we find out that there is no need to swap the values.