How to configure a Pyramid application behind Apache HTTPS Proxy - https

Question

I have a Pyramid application running on an internal server using mod_wsgi-express. The address internally is http://172.31.15.111:6543/myapp/
I want to make it available externally using Proxy and SSL. So I have the following Apache configuration:
<VirtualHost *:443>
ServerName myapp.org
ProxyRequests Off
ProxyPreserveHost On
# proxy pass /myapp to the internal:6543/myapp
ProxyPass /myapp http://172.31.15.111:6543/myapp/
ProxyPassReverse /myapp http://172.31.15.111:6543/myapp/
<Proxy *>
allow from all
</Proxy>
ProxyTimeout 1200
</VirtualHost>
The problem that I have is that when I use "request.route_url()" or "request.host_url" it resolves to HTTP and not to HTTPS.
I am using Waitress with the following configuration:
[server:main]
use = egg:waitress#main
listen = localhost:6543
I tried also:
[server:main]
use = egg:waitress#main
#listen = localhost:6543
host = 172.31.15.111
port = 6543
url_scheme = https
I also tried having request headers in apache:
RequestHeader set X-Forwarded-Port 443
RequestHeader set X-Forwarded-Scheme https
But I get the same result. Any idea how to resolve it is highly appreciated.