selectAll with Options fields fails
Using ``selectAll`` with Option field seems to throw an exception : 

```
/home/rcaillaud/.nimble/pkgs/norm-2.2.3/norm/postgres.nim(213) selectAll
/home/rcaillaud/.nimble/pkgs/norm-2.2.3/norm/postgres.nim(203) select
/home/rcaillaud/.nimble/pkgs/norm-2.2.3/norm/private/postgres/rowutils.nim(51) fromRow
/home/rcaillaud/.nimble/pkgs/norm-2.2.3/norm/private/postgres/rowutils.nim(38) fromRowPos
/home/rcaillaud/.nimble/pkgs/norm-2.2.3/norm/private/postgres/dbtypes.nim(65) to
/home/rcaillaud/.choosenim/toolchains/nim-1.4.4/lib/system/fatal.nim(49) sysFatal
Error: unhandled exception: 'i' is not accessible using discriminant 'kind' of type 'DbValue' [FieldDefect]
```

This is the case I was able to minimize : 
```nim
type
  Toy* = ref object of Model
    price*: float

  Pet* = ref object of Model
    species*: string
    favToy*: Toy

  Person* = ref object of Model
    name* {.unique.}: string
    pet*: Option[Pet]

proc newToy(): Toy =
  Toy(price: 0.0)

proc newPet(): Pet =
  Pet(species: "cat", favToy: newToy())

proc newPerson(): Person =
  Person(name: "Bob", pet: none(Pet))

proc readAllData() =
  var humans = @[newPerson()]
  withDb:
    db.selectAll(humans)   # Error


proc readData() =
  var human = newPerson()
  withDb:
    db.select(human, "id=$1", 1)
  # OK

proc createDb() =
  withDb:
    db.createTables(newToy())
    db.createTables(newPet())
    db.createTables(newPerson())

proc dropDb() =
  withDb:
    dropDb()

proc insertData() =
  var toy = Toy(price: rand(100.0))
  var buddy = Pet(species: "dog", favToy: toy)
  var h1 = Person(name: "Jean", pet: some(buddy))
  var h2 = Person(name: "Rachel", pet: none(Pet))

  withDb:
    db.insert(h1)
    db.insert(h2)

proc main() =
  # createDb()
  insertData() # Careful about double insertion
  readData()
  readAlldata()
  # dropDb()

when isMainModule:
  main()

```

Fails with both SQLite and Postgres backend. Changing ``Person`` to use ``Pet`` field instead of ``Option[Pet]`` removes the error.