DBValue do not convert to Field type Positive correctly.
continue with #27 
Thanks for your response, The code in #27 can be compiled.

continue to add code as follows:
```
withDb:
  let bobs = User.getMany(
    100,
    cond="name LIKE 'Bob%' ORDER BY age"
  )

  dump bobs
```
got build errors:
```
d:\sources\nim\main.nim(31) main
C:\Users\user\.nimble\pkgs\norm-1.0.16\norm\sqlite.nim(167) getMany
C:\Users\user\.nimble\pkgs\norm-1.0.16\norm\sqlite\rowutils.nim(105) getMany
D:\scoop\apps\nim\current\lib\system\fatal.nim(48) sysFatal
Error: unhandled exception: i is not accessible [FieldError]
Error: execution of an external program failed: 'd:\sources\nim\main.exe '
```
The problem could be line 105:
 `     obj.dot(field) = row[i].i.int`
 in C:\Users\hxk\.nimble\pkgs\norm-1.0.16\norm\sqlite\rowutils.nim,
the type of field age in User is Positive, but row[i] DBValue has no i field.