[Openid-specs-fapi] Issue #66: CustomerId == sub == x-fapi-customer-id: Should standardize as sub (openid/fapi)

[Nat Sakimura]

New issue 66: CustomerId == sub == x-fapi-customer-id: Should standardize as sub
https://bitbucket.org/openid/fapi/issues/66/customerid-sub-x-fapi-customer-id-should

Nat Sakimura:

In the spec, there are three ways to express semantically equal thing.

CustomerId == sub == x-fapi-customer-id

As an OIDC related spec, it should standardize on `sub`. 

Also, the http header value for this fixed value may not be trustworthy as it is not a secret and can be reproduced by anyone. So it should not be relied upon. Perhaps, it should be removed from the Security parts and moved to Part 4 to make sure that people does not misunderstand that this is a security feature.

Responsible: Nat