Preview mode show login page when admin and site have a different domain/subdomain

Steps to reproduce

1. Install clean eZ Platform 2.5
2. Configure site and admin site aware to using a different domain/subdomain 
3. Enter to Admin and try to preview the site 

Result

When trying to preview a site from the Page tab then display the login page instead of the site

Expected result

The preview displays the correct page of the site

Note
Edit mode working correctly, tested with our VCL for FASTLY.