Some external links don't behave as expected

[Farcasut]

Steps to Reproduce:

1. Create a diagram.
2. Add an external link without the https:// prefix, for example, www.google.com or just google.com.
3. Save the page.
4. Click on the link.

Expected Result:
Be redirected to google.com.

Actual Result:
The link redirects to an internal XWiki link in the form: xwikidomain/www.google.com.

[snazare]

fix future links only, not ones created already.