Convert userId to UTF8 before generating signature

__Problem description__
User ID packages have the userId as UTF8. In method openpgp.generate_key_pair we generate such a packet but later on when the signature for the key is generated userId is taken without UTF8 encoding. This leads to an invalid signature

__Example__
generate_key_pair(1, 1024, "한국어/조선말 äöü", "secret")
-> import into GPG fails