Insert scheme before *_proxy variable if needed

- wget seems to require fully valid URLs.
- cURL defaults to a HTTP proxy.
- Python's urllib falls back to the original request type (scheme).

We decide to insert the original scheme if missing.
If the http_proxy environment variable still contains true garbage, then
the module still returns garbage.

README.md

@@ -116,6 +116,7 @@ The following resources were used to determine the desired behavior:
 - cURL:
   https://curl.haxx.se/docs/manpage.html#ENVIRONMENT  
   https://github.com/curl/curl/blob/4af40b3646d3b09f68e419f7ca866ff395d1f897/lib/url.c#L4446-L4514  
+  https://github.com/curl/curl/blob/4af40b3646d3b09f68e419f7ca866ff395d1f897/lib/url.c#L4608-L4638  
 
 - wget: 
   https://www.gnu.org/software/wget/manual/wget.html#Proxies  
@@ -126,4 +127,5 @@ The following resources were used to determine the desired behavior:
   https://www.w3.org/Daemon/User/Proxies/ProxyClients.html  
 
 - Python's urllib:
+  https://github.com/python/cpython/blob/936135bb97fe04223aa30ca6e98eac8f3ed6b349/Lib/urllib/request.py#L755-L782  
   https://github.com/python/cpython/blob/936135bb97fe04223aa30ca6e98eac8f3ed6b349/Lib/urllib/request.py#L2444-L2479

index.js

@@ -39,7 +39,12 @@ function getProxyForUrl(url) {
     return '';  // Don't proxy URLs that match NO_PROXY.
   }
 
-  return getEnv(proto + '_proxy') || getEnv('all_proxy');
+  var proxy = getEnv(proto + '_proxy') || getEnv('all_proxy');
+  if (proxy && proxy.indexOf('://') === -1) {
+    // Missing scheme in proxy, default to the requested URL's scheme.
+    proxy = proto + '://' + proxy;
+  }
+  return proxy;
 }
 
 /**

test.js

@@ -104,8 +104,14 @@ describe('getProxyForUrl', function() {
     // Crazy values should be passed as-is. It is the responsibility of the
     // one who launches the application that the value makes sense.
     // TODO: Should we be stricter and perform validation?
-    env.HTTP_PROXY = 'Crazy \n!() { :: }';
-    testProxyUrl(env, 'Crazy \n!() { :: }', 'http://wow');
+    env.HTTP_PROXY = 'Crazy \n!() { ::// }';
+    testProxyUrl(env, 'Crazy \n!() { ::// }', 'http://wow');
+
+    // The implementation assumes that the HTTP_PROXY environment variable is
+    // somewhat reasonable, and if the scheme is missing, it is added.
+    // Garbage in, garbage out some would say...
+    env.HTTP_PROXY = 'crazy without colon slash slash';
+    testProxyUrl(env, 'http://crazy without colon slash slash', 'http://wow');
   });
 
   describe('https_proxy and HTTPS_PROXY', function() {
@@ -143,6 +149,19 @@ describe('getProxyForUrl', function() {
     testProxyUrl(env, 'http://priority', 'https://example');
   });
 
+  describe('all_proxy without scheme', function() {
+    var env = {};
+    env.ALL_PROXY = 'noscheme';
+    testProxyUrl(env, 'http://noscheme', 'http://example');
+    testProxyUrl(env, 'https://noscheme', 'https://example');
+
+    // The module does not impose restrictions on the scheme.
+    testProxyUrl(env, 'bogus-scheme://noscheme', 'bogus-scheme://example');
+
+    // But the URL should still be valid.
+    testProxyUrl(env, '', 'bogus');
+  });
+
   describe('no_proxy empty', function() {
     var env = {};
     env.HTTPS_PROXY = 'http://proxy';