dbValue for DateTime not valid in querying
Minimal example:

```nim
import times
import norm/[model, sqlite]
import sugar

type
  TestTime = ref object of Model
    time: DateTime

proc newTest(): TestTime = TestTime(time: now())

when isMainModule:
  var
    db = getDb()
    t = newTest().dup:
      db.select("TestTime.time <= ?", ?now())
  db.close()
```

Using the latest version installed by `nimble` (2.1.1).

Attempting to compile gives `Error: type mismatch: got <DateTime>`, listing all the other types defined in [`norm/private/sqlite/dbtypes`](https://github.com/moigagoo/norm/blob/develop/src/norm/private/sqlite/dbtypes.nim). Attempting to define my own conversion procs (i.e. copying the `DateTime` func from that file) gives an ambiguous call error:

`Error: ambiguous call; both dbtypes.dbValue(val: DateTime) [declared in ~/.nimble/pkgs/norm-2.1.1/norm/private/sqlite/dbtypes.nim(41, 6)] and database.dbValue(val: DateTime) [declared in ~/dev/test/src/test.nim(5, 8)] match for: (DateTime)`

I haven't had this issue declaring my own conversion methods for other types (e.g., `Time`). The issue doesn't crop up for inserting into the database or creating tables, just in `select` that I've found (so far).