feat(proxy): Allow to configure changeOrigin option of http-proxy

http-proxy has an option 'changeOrigin' (disabled by default) to rewrite
 the Host header in proxified requests wih the host from the target
 configuration.

Closes #1729

lib/middleware/proxy.js

@@ -44,6 +44,7 @@ var parseProxyConfig = function (proxies, config) {
       (proxyDetails.protocol === 'https:' ? '443' : '80')
     var https = proxyDetails.protocol === 'https:'
 
+    var changeOrigin = 'changeOrigin' in proxyConfiguration ? proxyConfiguration.changeOrigin : false
     var proxy = httpProxy.createProxyServer({
       target: {
         host: hostname,
@@ -52,6 +53,7 @@ var parseProxyConfig = function (proxies, config) {
         protocol: protocol
       },
       xfwd: true,
+      changeOrigin: changeOrigin,
       secure: config.proxyValidateSSL
     })