NHS: fix occasional bad numbers

[ChaoticBoredom]

As I've been working in this repo, I noticed the test for this code was being skipped. On investigation, it was because this generator was not truly accurate, and the test would occasionally fail (as it should have)
The generator would create a final digit for the generated number, but did not handle the edge cases when this 'check digit' was 10 or 11.
In the case when the check digit is 10, the entire number is invalid. In this situation, I subtracted one from the generated number which would then be guaranteed to be a valid number.
In the case when the check digit is 11, it should be returned as a 0, not as 11.
Rules were read from the wikipedia page that was linked in the original PR that introduced this faker.
These changes have tests, and the original test that set me onto this problem is no longer omitted, and should be consistently passing now.

[vbrazo]

I investigated this failing test but I couldn't fix it and omitted the test, so other contributors could see it and fix whenever they feel comfortable.

Thanks for the explanation in the description the PR 👍