Problem with link field (internal and external)

Problem/Motivation

Link can be both internal (with autocomplete) and external.
If we try to edit field that had internal url to set external url, the link is automatically deleted on submit because the values does not match.

Steps to reproduce

Create a content with link field (internal and external)
Create a node with internal url (via autocomplete)
Edit this node and set external url, the field is empty on submit

Proposed resolution

I wrote a patch to fix it.
When the value does not match with the real data attributes, instead of deleting it, we keep the real value.
I fixed it for classic submit and ajax ones.