Normalize the representation of Scope.content data in MongoModulestore FieldData

Without this, the new 'items' field in the course_info xmodule caused
bizarre behavior. If you added the text 'items' to a course_info
section, then the LMS would throw a 500, because the field name isn't
'data', but the stored data is a string, leading to the check of
`key.field_name in self._data`, where key.field_name is the string 'items',
and self._data is the content string.