Default creators value should be unicode

(repeat of #93, this time without broken history)

Stumbled on this whilst tinkering with transmogrify.dexterity - if no value is supplied for a field, the schemaupdater tries to get the default value for a field then set it to that, similar to the provided unit test. If the default value for creators isn't unicode, then it fails with a validation error.

I'm not entirely sure why schemaupdater does this mind, the code seems a bit odd. But ensuring the default value passes validation seems reasonable, if somewhat academic.