Convert DateTime to Time before quoting

`DateTime` does not respond to `getlocal`, so the current quoting is wrong for DateTimes when the database is in local time: https://github.com/github/rails/blob/3-0-github/activerecord/lib/active_record/connection_adapters/abstract/quoting.rb#L65-66

```
[34] pry(#<GitHubSQLContext>)> datetime_local
=> Wed, 14 May 2014 16:17:30 +1000
[35] pry(#<GitHubSQLContext>)> datetime_utc
=> Wed, 14 May 2014 06:17:30 +0000
[36] pry(#<GitHubSQLContext>)> ActiveRecord::Base.connection.quote(datetime_local)
=> "'2014-05-14 16:17:30'"
[37] pry(#<GitHubSQLContext>)> ActiveRecord::Base.connection.quote(datetime_utc)
=> "'2014-05-14 06:17:30'"
```

cc @github/timezones @github/rails3