incorrect !seen results when username contains capital letters

[tangentstorm]

It appears seen.py records an invalid timestamp when a username contains capital letters.

Example 1: a known user's timestamp is corrupted when tamper sees the username with a different capitalization:

14:06:24 TangentStorm !seen darkf
14:06:25 tamper darkf last seen 12 hours ago
14:06:28 DaRkF sonahuaoestnuh
14:06:30 TangentStorm !seen darkf
14:06:31 tamper DaRkF last seen 3 months ago

Example 2: a channel regular who happens to capitalize his nick does not appear in the database at all:

14:10:46 tangentstorm !seen Amadiro
14:10:46 tamper I have not seen Amadiro

(Amadiro was in fact in the channel at that very moment)