DxBuilder: Fix encoding problem when filling default value for "owners" field

Fixes #23 

The default value adapter for the ``IOwnership.owners`` field returns a tuple of byte strings, but unicodes are required.

Although it is not correct it is not a problem in the "real" world, since the default value adapter is used for filling the form field, which is converted to unicodes correctly when submitted.
Therefore I decided to do a hack which fixes this issue for exactly this field. I've not done it in a more generic way because our custom fields should have better default value adapters which use unicodes.

@elioschmutz @deiferni this fixes the issues you've had with
```
WrongContainedType: ([WrongType('test_user_1_', <type 'unicode'>, '')], 'creators')
```