Always look up keys by canonical email address

The original code did first try to look up the key by
non-canonical email address (e.g. 'Joe User <joe@user.com>') which
can lead to wrong key results if there are keys with the same name but
different email addresses. The fallback lookup with canonical address
wouldn't be triggered in this case, because the first lookup did return
a result (although a wrong one).
Now we only do one lookup with the canonical email address.