Expand descriptors with defaults

[roll]

Overview

When you get descriptor you need to apply defaults from specs (like default type for field or default profile for datapackage).

Tasks

• port this functions https://github.com/frictionlessdata/datapackage-py/blob/master/datapackage/helpers.py#L138-L170
• re-use it in DataPackage, Resource and validate to dereference descriptor (see Py/Js implementation)