Forward proxy of HTTPS by Apache HTTPD

I wanted to set up an forward proxy server for a web site that uses
both HTTPS and HTTP protocols.
Okay it is well documented in
<http://httpd.apache.org/docs/2.2/en/mod/mod_proxy.html>.

One thing I had to find by trial and error is that https connection is
implemented by CONNECT method of HTTP and is not represented directly
in <Proxy> directive.
Instead we have to match URI in form "proxy:host:443".

# for HTTP
<Proxy http://{ORIGIN-SERVER}/*>
Order deny,allow
Allow from {CLIENT-IP}
</Proxy>

# for HTTPS
<Proxy proxy:{ORIGIN-SERVER}:443>
Order deny,allow
Allow from {CLIENT-IP}
</Proxy>