input: Write a python function to find the sum of all divisors of a given number.
  assert sum_divisors(6) == 6
  assert sum_divisors(28) == 28
  assert sum_divisors(12) == 16
output: def sum_divisors(n):
      total = 0
      for i in range(1, n + 1):
          if n % i == 0:
              total += i
      return total