Link content type serializer for external url

[yurj]

Hi!

after merging plone/plone.app.contenttypes#711 the link serializer will return the correct path for Link using ${navigation_root_url} or ${portal_url} in external url.

Code:

plone.restapi/src/plone/restapi/serializer/dxfields.py

Line 161 in 4f3bb4f

path = replace_link_variables_by_paths(context=self.context, url=value)