Modified the ad-hoc proxy lookup to use _getKey(), rather than the

error-prone in-line method.

_getKey() will look for options.target as well as
options.host:options.port, and so is useful for a segmented
proxy server where the destination proxies are not constructed before
first references.

lib/node-http-proxy/routing-proxy.js

@@ -180,8 +180,8 @@ RoutingProxy.prototype.proxyRequest = function (req, res, options) {
     options.host = location.host;
   }
 
-  var key = options.host + ':' + options.port,
-      proxy;
+  var key = this._getKey(options),
+    proxy;
 
   if (!this.proxies[key]) {
     this.add(options);
@@ -218,8 +218,8 @@ RoutingProxy.prototype.proxyWebSocketRequest = function (req, socket, head, opti
     options.host = location.host;
   }
 
-  var key = options.host + ':' + options.port,
-      proxy;
+  var key = this._getKey(options),
+    proxy;
 
   if (!this.proxies[key]) {
     this.add(options);