Adding a way to disable all proxy processing

When the proxy field in configuration is === false all proxy processing is
disabled. This specifically disable the 'http_proxy' environment variable
handling.

Fixes #635
Related to #434

README.md

@@ -303,6 +303,8 @@ These are the available config options for making requests. Only the `url` is re
   httpsAgent: new https.Agent({ keepAlive: true }),
 
   // 'proxy' defines the hostname and port of the proxy server
+  // On node the proxy can also come from the 'http_proxy' environment variable.
+  // Use false to disable all proxy handling.
   // `auth` indicates that HTTP Basic auth should be used to connect to the proxy, and supplies credentials.
   // This will set an `Proxy-Authorization` header, overwriting any existing `Proxy-Authorization` custom headers you have set using `headers`.
   proxy: {
@@ -526,7 +528,7 @@ In a browser, you can use the [`URLSearchParams`](https://developer.mozilla.org/
 var params = new URLSearchParams();
 params.append('param1', 'value1');
 params.append('param2', 'value2');
-axios.post('/foo', params); 
+axios.post('/foo', params);
 ```
 
 > Note that `URLSearchParams` is not supported by all browsers, but there is a [polyfill](https://github.com/WebReflection/url-search-params) available (make sure to polyfill the global environment).

lib/adapters/http.js

@@ -82,7 +82,7 @@ module.exports = function httpAdapter(config) {
     };
 
     var proxy = config.proxy;
-    if (!proxy) {
+    if (!proxy && proxy !== false) {
       var proxyEnv = protocol.slice(0, -1) + '_proxy';
       var proxyUrl = process.env[proxyEnv] || process.env[proxyEnv.toUpperCase()];
       if (proxyUrl) {

test/unit/adapters/http.js

@@ -286,6 +286,23 @@ module.exports = {
     });
   },
 
+  testHTTPProxyDisabled: function(test) {
+    // set the env variable
+    process.env.http_proxy = 'http://does-not-exists.example.com:4242/';
+
+    server = http.createServer(function(req, res) {
+      res.setHeader('Content-Type', 'text/html; charset=UTF-8');
+      res.end('123456789');
+    }).listen(4444, function() {
+      axios.get('http://localhost:4444/', {
+          proxy: false
+        }).then(function(res) {
+          test.equal(res.data, '123456789', 'should not pass through proxy');
+          test.done();
+        });
+    });
+  },
+
   testHTTPProxyEnv: function(test) {
     server = http.createServer(function(req, res) {
       res.setHeader('Content-Type', 'text/html; charset=UTF-8');