Fix problem in ES validating NIE numbers starting with Y and Z.

[danielmaxx]

Some time ago this ticket was opened but not correctly fixed.

The checksum should be calculated by prepending the index of the first letter. More reference can be found here: https://es.wikipedia.org/wiki/N%C3%BAmero_de_identificaci%C3%B3n_fiscal#D.C3.ADgito_de_control_para_n.C3.BAmeros_de_DNI_o_NIE

[benkonrath]

This looks great, thanks! Can you just add a couple of housekeeping items:

• Add your name to docs/authors.rst if it's not already there.
• Add an entry to the docs/changelog.rst

Thanks.

[danielmaxx]

Done! I hope everything is fine now.

Many thanks for the kind help!

[benkonrath]

Looks good. Can you just rebase from master and squash the commits? Thanks!

[benkonrath]

I took care of the rebase and squash. I also fixed a small issue with changelog. Thanks for your help. Please do send in more fixes if you find anymore problems.

Fixed via e37786d.