fix: Small bug in potter Kata

[voghDev]

Hi!

After summoning this repo, I've found a small bug in the potter kata. The problem was that groupBooks was returning a map with a structure like:

{
  "1": 1,
  "2": 2,
  "3": 1,
  ... 
}

and after decreasing the counter inside removeDiscountedBooks, we could have

{
  "1": 0,
  "2": 1,
  "3": 0,
  ... 
}

so, when obtaining books[0], we could get a 0 instead of a 1, which calculates a wrong price. Instead of getting 8 * 1 = 8, we would get 8 * 0 = 0.

One possible solution would be to remove the entries from the Map where the value is 0 in removeDiscountedBooks. I wrote it in this PR. I Hope you find it useful :smiley:

I've added a test that makes the current master branch fail. If you remove the .filter() in this PR, you will see that the test expects 38 and gets 30.