HAL JSON uses JSON, which uses UTF-8, but properties of ERP5 documents
does not enforce any specific encoding, so if we happen to have
documents with string properties that are not UTF-8 compatible, they
could not be queried with HAL JSON (json.dumps was raising UnicodeError)

This pre-process field default properties and document title to make
sure they can be serialized as json. This assumes that configuration
(form definition, actions, ...) will be valid UTF-8, but just does extra
effort to make sure "broken" data can be displayed without error.

/reviewed-on !928