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/dispatchRequest.js

@@ -27,7 +27,8 @@ module.exports = function dispatchRequest(config) {
   config.headers = config.headers || {};
 
   // Transform request data
-  config.data = transformData(
+  config.data = transformData.call(
+    config,
     config.data,
     config.headers,
     config.transformRequest
@@ -53,7 +54,8 @@ module.exports = function dispatchRequest(config) {
     throwIfCancellationRequested(config);
 
     // Transform response data
-    response.data = transformData(
+    response.data = transformData.call(
+      config,
       response.data,
       response.headers,
       config.transformResponse
@@ -66,7 +68,8 @@ module.exports = function dispatchRequest(config) {
 
       // Transform response data
       if (reason && reason.response) {
-        reason.response.data = transformData(
+        reason.response.data = transformData.call(
+          config,
           reason.response.data,
           reason.response.headers,
           config.transformResponse