This change corrects verification of certifications over User Attributes
(such as photos).
Before this change the code did not differentiate between User IDs and
User Attributes as both of them were stored in `data.userid` [0] and
incorrectly used the User ID constant (0xB4) for both cases.
This change fixes the bug by storing User IDs in `userId` property and
User Attributes in `userAttribute` property. The check for property
existence has been modified to avoid comparisons with `undefined` as the
`User` class sets `null` for not assigned packets instead of
`undefined`.
Only data structures for signing and verification were modified and not
the properties used in the `User` class.
[0]: 11b2d2de3c/src/key.js (L872)
19e3c344fd