Write the recursive function sum_integer that takes an integer n and repeatedly calculates the sum of the input digits until the result is less than 10.
sum_integer
n
sum_integer(105377312) -> 2 sum_integer(99999) -> 9
Test 1    
Test 2    
Test 3    
Test 4    
Test 5    
Test 6    
Test 7    
Test 8