The reason you are seeing this change is that we removed "trim_urls" from the core filter set. You can add it back with
ModPagespeedEnableFilters trim_urls
The reason that we removed it was that we were having numerous reports of mod_pagespeed breaking HTML that is served via an Ajax request, where mod_pagespeed had the wrong idea of what base URL the browser was going to interpret the code with. If this is not an issue for you then it should be safe to add back in "trim_urls".
Another command-sequence you might want to be aware of is
ModPagespeedEnableFilters rewrite_domains
You might not need it on your site because the previous situation worked fine for you, where mod_pagespeed thought it was serving http:// to the browser but trim_urls made the point moot.
But this pair of commands might come in handy if you do need to move other absolutely-specified resources onto https://. This command will work even for many URLs found in HTML files that are not otherwise rewritten or optimized by mod_pagespeed.
-Josh