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);

README.md

@@ -458,6 +458,19 @@ These are the available config options for making requests. Only the `url` is re
   // - Node only (XHR cannot turn off decompression)
   decompress: true // default
 
+  // transitional options for backward compatibility that may be removed in the newer versions
+  transitional: {
+    // silent JSON parsing mode
+    // `true`  - ignore JSON parsing errors and set response.data to null if parsing failed (old behaviour)
+    // `false` - throw SyntaxError if JSON parsing failed (Note: responseType must be set to 'json')
+    silentJSONParsing: true; // default value for the current Axios version
+
+    // try to parse the response string as JSON even if `resposeType` is not 'json'
+    forcedJSONParsing: true;
+    
+    // throw ETIMEDOUT error instead of generic ECONNABORTED on request timeouts
+    clarifyTimeoutError: false;
+  }
 }
 ```