JSON improvements: throw if JSON parsing failed; number, boolean can be passed directly as payload for encoding to JSON #2613, #61, #907 (#3688)

* Draft

* Added support for primitive types to be converted to JSON if the request Content-Type is 'application/json';
Added throwing SyntaxError if JSON parsing failed and responseType is json;
Added transitional option object;
Added options validator to assert transitional options;
Added transitional option `silentJSONParsing= true` for backward compatibility;
Updated README.md;
Updated typings;

* Fixed isOlderVersion helper;
Fixed typo;
Added validator.spec.js;

* Added forcedJSONParsing transitional option #2791

* `transformData` is now called in the default configuration context if the function context is not specified (for tests compatibility);

* Added `transitional.clarifyTimeoutError` to throw ETIMEDOUT error instead of generic ECONNABORTED on request timeouts;
Added support of onloadend handler if available instead of onreadystatechange;
Added xhr timeout test;
Fixed potential bug of xhr adapter with proper handling timeouts&errors (FakeXMLHTTPRequest failed to handle timeouts);

lib/core/transformData.js

@@ -1,6 +1,7 @@
 'use strict';
 
 var utils = require('./../utils');
+var defaults = require('./../defaults');
 
 /**
  * Transform the data for a request or a response
@@ -11,9 +12,10 @@ var utils = require('./../utils');
  * @returns {*} The resulting transformed data
  */
 module.exports = function transformData(data, headers, fns) {
+  var context = this || defaults;
   /*eslint no-param-reassign:0*/
   utils.forEach(fns, function transform(fn) {
-    data = fn(data, headers);
+    data = fn.call(context, data, headers);
   });
 
   return data;