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);

test/specs/transform.spec.js

@@ -41,6 +41,90 @@ describe('transform', function () {
     });
   });
 
+  it('should throw a SyntaxError if JSON parsing failed and responseType is "json" if silentJSONParsing is false',
+    function (done) {
+      var thrown;
+
+      axios({
+        url: '/foo',
+        responseType: 'json',
+        transitional: {silentJSONParsing: false}
+      }).then(function () {
+        done(new Error('should fail'));
+      }, function (err) {
+        thrown = err;
+      });
+
+      getAjaxRequest().then(function (request) {
+        request.respondWith({
+          status: 200,
+          responseText: '{foo": "bar"}' // JSON SyntaxError
+        });
+
+        setTimeout(function () {
+          expect(thrown).toBeTruthy();
+          expect(thrown.name).toContain('SyntaxError');
+          expect(thrown.message).toContain('JSON');
+          done();
+        }, 100);
+      });
+    }
+  );
+
+  it('should send data as JSON if request content-type is application/json', function (done) {
+    var response;
+
+    axios.post('/foo', 123, {headers: {'Content-Type': 'application/json'}}).then(function (_response) {
+      response = _response;
+    }, function (err) {
+      done(err);
+    });
+
+    getAjaxRequest().then(function (request) {
+      request.respondWith({
+        status: 200,
+        responseText: ''
+      });
+
+      setTimeout(function () {
+        expect(response).toBeTruthy();
+        expect(request.requestHeaders['Content-Type']).toBe('application/json');
+        expect(JSON.parse(request.params)).toBe(123);
+        done();
+      }, 100);
+    });
+  });
+
+  it('should not assume JSON if responseType is not `json`', function (done) {
+    var response;
+
+    axios.get('/foo', {
+      responseType: 'text',
+      transitional: {
+        forcedJSONParsing: false
+      }
+    }).then(function (_response) {
+      response = _response;
+    }, function (err) {
+      done(err);
+    });
+
+    var rawData = '{"x":1}';
+
+    getAjaxRequest().then(function (request) {
+      request.respondWith({
+        status: 200,
+        responseText: rawData
+      });
+
+      setTimeout(function () {
+        expect(response).toBeTruthy();
+        expect(response.data).toBe(rawData);
+        done();
+      }, 100);
+    });
+  });
+
   it('should override default transform', function (done) {
     var data = {
       foo: 'bar'