SSL errors on Heroku (Mixed contents)
IMO $base_url in settings.php should work 

Alternatively you can try 

RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://example.com/$1 [R,L]


or

RewriteEngine On
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]