postgres does not like nil in datetime field

fix in fixtures/messages.yml

```
PG::InvalidDatetimeFormat: ERROR:  invalid input syntax for type timestamp: "nil"
LINE 1: ... (1, 3, 2, 'f', 'f', 'Test message', 'Test body', 'nil', '20...
                                                             ^
: INSERT INTO "messages" ("id", "sender_id", "recipient_id", "sender_deleted", "recipient_deleted", "subject", "body", "read_at", "created_at", "updated_at") VALUES (1, 3, 2, 'f', 'f', 'Test message', 'Test body', 'nil', '2014-03-11 01:06:30.000000', '2014-03-11 01:06:30.000000')
```