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>

lib/core/mergeConfig.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var utils = require('../utils');
+import utils from '../utils.js';
 
 /**
  * Config-specific merge-function which creates a new config-object
@@ -11,10 +11,10 @@ var utils = require('../utils');
  *
  * @returns {Object} New object resulting from merging config2 to config1
  */
-module.exports = function mergeConfig(config1, config2) {
+export default function mergeConfig(config1, config2) {
   // eslint-disable-next-line no-param-reassign
   config2 = config2 || {};
-  var config = {};
+  const config = {};
 
   function getMergedValue(target, source) {
     if (utils.isPlainObject(target) && utils.isPlainObject(source)) {
@@ -61,7 +61,7 @@ module.exports = function mergeConfig(config1, config2) {
     }
   }
 
-  var mergeMap = {
+  const mergeMap = {
     'url': valueFromConfig2,
     'method': valueFromConfig2,
     'data': valueFromConfig2,
@@ -92,10 +92,10 @@ module.exports = function mergeConfig(config1, config2) {
   };
 
   utils.forEach(Object.keys(config1).concat(Object.keys(config2)), function computeConfigValue(prop) {
-    var merge = mergeMap[prop] || mergeDeepProperties;
-    var configValue = merge(prop);
+    const merge = mergeMap[prop] || mergeDeepProperties;
+    const configValue = merge(prop);
     (utils.isUndefined(configValue) && merge !== mergeDirectKeys) || (config[prop] = configValue);
   });
 
   return config;
-};
+}