Improved FormData support; (#4448)

* Fixed isFormData predicate;
Added support for automatic object serialization to FormData if `Content-Type` is `multipart/form-data`;
Added support for FormData to be overloaded using `config.env.FormData` option;
Added support for FormData in node.js environment through `form-data` package;

* Added the `form-data` package as a dependency for the server build;
Added tests for FormData payload;

* Added FormData automatic serialization section;
Refactored cancellation section;

* Reworked toFormData helper;
Expose toFormData helper as a static method;
Refactored transform request;
Added kindOf, kindOfTest, endsWith, isTypedArray util;
Refactored utils.js to use kindOf for tests;

* Fixed isFormData predicate; (#4413)

Added support for automatic object serialization to FormData if `Content-Type` is `multipart/form-data`;
Added support for FormData to be overloaded using `config.env.FormData` option;
Added support for FormData in node.js environment using `form-data` package;

(cherry picked from commit 73e3bdb883)

* Added shortcut methods `postForm`, `putForm`, `patchForm` to submit a Form;
Added ability to submit FileList object as a FormData;
Updated README.md;

* Updated README.md;

lib/core/Axios.js

@@ -136,13 +136,23 @@ utils.forEach(['delete', 'get', 'head', 'options'], function forEachMethodNoData
 
 utils.forEach(['post', 'put', 'patch'], function forEachMethodWithData(method) {
   /*eslint func-names:0*/
-  Axios.prototype[method] = function(url, data, config) {
-    return this.request(mergeConfig(config || {}, {
-      method: method,
-      url: url,
-      data: data
-    }));
-  };
+
+  function generateHTTPMethod(isForm) {
+    return function httpMethod(url, data, config) {
+      return this.request(mergeConfig(config || {}, {
+        method: method,
+        headers: isForm ? {
+          'Content-Type': 'multipart/form-data'
+        } : {},
+        url: url,
+        data: data
+      }));
+    };
+  }
+
+  Axios.prototype[method] = generateHTTPMethod();
+
+  Axios.prototype[method + 'Form'] = generateHTTPMethod(true);
 });
 
 module.exports = Axios;