install: support HTTPS_PROXY

Fixes: nodejs#1749

lib/install.js

@@ -14,6 +14,7 @@ var fs = require('graceful-fs')
   , os = require('os')
   , tar = require('tar')
   , path = require('path')
+  , url = require('url')
   , crypto = require('crypto')
   , log = require('npmlog')
   , semver = require('semver')
@@ -413,11 +414,11 @@ function install (fs, gyp, argv, callback) {
 
 }
 
-function download (gyp, env, url) {
-  log.http('GET', url)
+function download (gyp, env, uri) {
+  log.http('GET', uri)
 
   var requestOpts = {
-      uri: url
+      uri
     , headers: {
         'User-Agent': 'node-gyp v' + gyp.version + ' (node ' + process.version + ')'
       }
@@ -429,10 +430,24 @@ function download (gyp, env, url) {
   }
 
   // basic support for a proxy server
-  var proxyUrl = gyp.opts.proxy
-              || env.http_proxy
-              || env.HTTP_PROXY
-              || env.npm_config_proxy
+  var proxyUrl = gyp.opts.proxy;
+
+  // prefer HTTPS proxy if contacting an HTTPS uri
+  if (url.parse(uri).protocol === 'https:') {
+    if (!proxyUrl) {
+      proxyUrl = env.https_proxy
+              || env.HTTPS_PROXY
+              || env.npm_config_https_proxy
+    }
+  }
+
+  // but fallback to an HTTP proxy as needed
+  if (!proxyUrl) {
+    proxyUrl = env.http_proxy
+            || env.HTTP_PROXY
+            || env.npm_config_proxy
+  }
+
   if (proxyUrl) {
     if (/^https?:\/\//i.test(proxyUrl)) {
       log.verbose('download', 'using proxy url: "%s"', proxyUrl)