Two fixes have been added in 2.5.19:
Due to a merge where two multisite filters were not uncommented (uncommented code should have replaced commented out code) the 2.5.18 release did not contain the fix anymore the made sure WordPress returns the correct protocol. In 2.5.19 these filters are active again.
Mixed content fixer
Additionally a constant was added to allow for another hook to initialize the mixed content fixer on.
If this constant is defined and true, the Really Simple SSL mixed content fixer will fire on the init hook instead of the template_redirect hook.
The template redirect hook is the preferred way as this hook only fires when an actual page request is done, which prevents issues with json and XMLRPC requests and downloads, but in some situations, when a plugin conflict causes the mixed content fixer not to run, it’s now still possible to fall back to the init hook.
to your wp-config.php