/*
 * call-seq:
 *   easy.proxy_url = string                          => string
 *
 * Set the URL of the HTTP proxy to use for subsequent calls to +perform+.
 * The URL should specify the the host name or dotted IP address. To specify
 * port number in this string, append :[port] to the end of the host name.
 * The proxy string may be prefixed with [protocol]:// since any such prefix
 * will be ignored. The proxy's port number may optionally be specified with
 * the separate option proxy_port .
 *
 * When you tell the library to use an HTTP proxy, libcurl will transparently
 * convert operations to HTTP even if you specify an FTP URL etc. This may have
 * an impact on what other features of the library you can use, such as
 * FTP specifics that don't work unless you tunnel through the HTTP proxy. Such
 * tunneling is activated with proxy_tunnel = true.
 *
 * libcurl respects the environment variables *http_proxy*, *ftp_proxy*,
 * *all_proxy* etc, if any of those is set. The proxy_url option does however
 * override any possibly set environment variables.
 *
 * Starting with libcurl 7.14.1, the proxy host string given in environment
 * variables can be specified the exact same way as the proxy can be set with
 * proxy_url, including protocol prefix (http://) and embedded user + password.
  */
static VALUE ruby_curl_easy_proxy_url_set(VALUE self, VALUE proxy_url) {