Axios ES2017 (#4787)

* Added AxiosHeaders class;

* Fixed README.md href;

* Fixed a potential bug with headers normalization;

* Fixed a potential bug with headers normalization;
Refactored accessor building routine;
Refactored default transforms;
Removed `normalizeHeaderName` helper;

* Added `Content-Length` accessor;
Added missed `has` accessor to TS types;

* Added `AxiosTransformStream` class;
Added progress capturing ability for node.js environment;
Added `maxRate` option to limit the data rate in node.js environment;
Refactored event handled by `onUploadProgress` && `onDownloadProgress` listeners in browser environment;
Added progress & data rate tests for the http adapter;
Added response stream aborting test;
Added a manual progress capture test for the browser;
Updated TS types;
Added TS tests;
Refactored request abort logic for the http adapter;
Added ability to abort the response stream;

* Remove `stream/promises` & `timers/promises` modules usage in tests;

* Use `abortcontroller-polyfill`;

* Fixed AxiosTransformStream dead-lock in legacy node versions;
Fixed CancelError emitting in streams;

* Reworked AxiosTransformStream internal logic to optimize memory consumption;
Added throwing an error if the request stream was silently destroying (without error) Refers to #3966;

* Treat the destruction of the request stream as a cancellation of the request;
Fixed tests;

* Emit `progress` event in the next tick;

* Initial refactoring;

* Refactored Mocha tests to use ESM;

* Refactored Karma tests to use rollup preprocessor & ESM;
Replaced grunt with gulp;
Improved dev scripts;
Added Babel for rollup build;

* Added default commonjs package export for Node build;
Added automatic contributors list generator for package.json;

Co-authored-by: Jay <jasonsaayman@gmail.com>

test/specs/headers.spec.js

@@ -1,7 +1,7 @@
 function testHeaderValue(headers, key, val) {
-  var found = false;
+  let found = false;
 
-  for (var k in headers) {
+  for (const k in headers) {
     if (k.toLowerCase() === key.toLowerCase()) {
       found = true;
       expect(headers[k]).toEqual(val);
@@ -28,12 +28,12 @@ describe('headers', function () {
   });
 
   it('should default common headers', function (done) {
-    var headers = axios.defaults.headers.common;
+    const headers = axios.defaults.headers.common;
 
     axios('/foo');
 
     getAjaxRequest().then(function (request) {
-      for (var key in headers) {
+      for (const key in headers) {
         if (headers.hasOwnProperty(key)) {
           expect(request.requestHeaders[key]).toEqual(headers[key]);
         }
@@ -43,12 +43,12 @@ describe('headers', function () {
   });
 
   it('should add extra headers for post', function (done) {
-    var headers = axios.defaults.headers.common;
+    const headers = axios.defaults.headers.common;
 
     axios.post('/foo', 'fizz=buzz');
 
     getAjaxRequest().then(function (request) {
-      for (var key in headers) {
+      for (const key in headers) {
         if (headers.hasOwnProperty(key)) {
           expect(request.requestHeaders[key]).toEqual(headers[key]);
         }
@@ -72,15 +72,17 @@ describe('headers', function () {
         'x-header-a': null,
         'x-header-b': undefined
       }
+    }).catch(function (err) {
+      done(err);
     });
 
     getAjaxRequest().then(function (request) {
-      testHeaderValue(request.requestHeaders, 'Content-Type', null);
-      testHeaderValue(request.requestHeaders, 'x-header-a', null);
+      testHeaderValue(request.requestHeaders, 'Content-Type', undefined);
+      testHeaderValue(request.requestHeaders, 'x-header-a', undefined);
       testHeaderValue(request.requestHeaders, 'x-header-b', undefined);
       testHeaderValue(request.requestHeaders, 'x-header-c', 'c');
       done();
-    });
+    }).catch(done);
   });
 
   it('should use application/json when posting an object', function (done) {