Fix Company.luhn_algorithm and add missing tests.

[01max]

There is some issues with Faker::Company.luhn_algorithm(number) :

• Generates non "luhn-compliant" strings for even length number
• Not tested

The problem was that luhn_algorithm didn't reverse the number submitted argument before splitting it (as it should).

This forgotten reverse resulted in the generation of non "luhn-compliant" strings for even length number.

index	0	1	2	3	4	5
odd_base	9	8	4	5	6
reserved_odd_base	6	5	4	8	9
selected numbers for odd_base *2 by luh_algorithm	9		4		6
right numbers for odd_base *2 by luh_algorithm	6		4		9
even_base	8	7	6	9	4	3
reserved_even_base	3	4	9	6	7	8
selected numbers for even_base *2 by luh_algorithm	8		6		4
right numbers for even_base *2 by luh_algorithm	3		9		7

No issue for odd length arguments ([9,4,6] vs [6,4,9]), only with even ([8,6,4] vs [3,9,7]).

This commit fixes the issue and add test_luhn_algorithm to avoid future issues.

[01max]

I rebased on master since some new commits added conflicts since my PR has been submitted.

Can someone review this ?

[vbrazo]

Thanks for adding the missing test. I just asked you to rename the a variable because a doesn't mean anything in that context. I tested locally. Overall looks good. Thanks @01max

[01max]

I improved the last commit.

Thanks for the review @vbrazo !