Install: Improves code to get proxy.

* Credit: @xzyfer.
* Addresses #588.

Author: am11

scripts/install.js

@@ -43,16 +43,11 @@ function download(url, dest, cb) {
  */
 
 function getProxy() {
-  var result;
-
-  ['https-proxy', 'proxy', 'http-proxy'].map(function(config) {
+  var result = ['https-proxy', 'proxy', 'http-proxy'].filter(function(config) {
     var proxy = exec('npm config get ' + config, {silent: true});
     var output = proxy.output.trim();
 
-    if (proxy.code === 0 && output !== 'undefined' && output !== 'null') {
-      result = proxy.output;
-      return;
-    }
+    return proxy.code === 0 && validateProxyUrl(output);
   });
 
   if (result) {
@@ -63,6 +58,19 @@ function getProxy() {
   return env.HTTPS_PROXY || env.https_proxy || env.HTTP_PROXY || env.http_proxy;
 }
 
+/**
+ * Validates Proxy URL
+ *
+ * @param {String} url
+ * @api private
+ */
+
+function validateProxyUrl(url) {
+  return output !== 'undefined' &&
+         output !== 'null' &&
+         url === require('url').parse(url);
+}
+
 /**
  * Check if binaries exists
  *