Fix lt & lteq don't accept most of values.

`lt` & `lteq` predicates just accept number values now. Add the same way as `gt` & `gteq` in order to make it working for all values.

Examples:

```ruby
table = Table.new[:user]
mgr = table.project relation[:id]
mgr.where table[:created_at].lt(Time.now)

# Before fixed, it raises an error.
mgr.to_sql
```

Thanks for reviewing this.