Split the user ssh keys by newline, not the characters "\n"

before:
GET /user.keys

ssh-rsa ...\nssh-rsa ...\nssh-rsa ...

after:
GET /user.keys

ssh-rsa ...
ssh-rsa ...
sha-rsa ...
Merge pull request #6297 from jojosch/user-public-ssh-keys-newline

Split the user ssh keys by newline, not the characters "\n"