If I try to enter a Google+ page I get an error 403. I CAN enter in the Google+ user number but not the full URL itself. The only way I can get this plugin to work for rel=author is to hack the code to predefine the URL with https and then have the variable enter the Google+ ID - which is probably the way it should have been designed, IMHO. Still great work. No... better than great. This is an incredible plugin.
In class-frontend.php I changed the line to:
echo '<link rel="author" href="https://plus.google.com/u/0/' . $gplus . '/posts"/>' . "\n";
Works like a charm.