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);

index.d.ts

@@ -41,6 +41,12 @@ export type ResponseType =
   | 'text'
   | 'stream'
 
+export interface TransitionalOptions{
+  silentJSONParsing: boolean;
+  forcedJSONParsing: boolean;
+  clarifyTimeoutError: boolean;
+}
+
 export interface AxiosRequestConfig {
   url?: string;
   method?: Method;
@@ -71,6 +77,7 @@ export interface AxiosRequestConfig {
   proxy?: AxiosProxyConfig | false;
   cancelToken?: CancelToken;
   decompress?: boolean;
+  transitional?: TransitionalOptions
 }
 
 export interface AxiosResponse<T = any>  {