Fix question 17

I fixed two bugs in question 17:

1. You forgot the assignment when applying the bitwise operators. Should be ```<<=``` and not ```<<```. Notice how slow is ```my_division(1e7,1)```.

2. As soon as you fix the first problem, it will fail this test: 
  ```python
  def test_by_one(self):
          self.assertEqual(my_division(10, 1), 10)
  ```
  Removing lines 24 and 25 fixes this.