Added toJSONObject util; (#5247)

Fixed AxiosError.toJSON method to avoid circular references;

Co-authored-by: Jay <jasonsaayman@gmail.com>

test/unit/utils/utils.js

@@ -23,4 +23,30 @@ describe('utils', function (){
       assert.equal(utils.isFormData(new FormData()), true);
     });
   });
+
+  describe('toJSON', function (){
+    it('should convert to a plain object without circular references', function () {
+      const obj= {a: [0]}
+      const source = {x:1, y:2, obj};
+      source.circular1 = source;
+      obj.a[1] = obj;
+
+      assert.deepStrictEqual(utils.toJSONObject(source), {
+        x: 1, y:2, obj: {a: [0]}
+      });
+    });
+
+    it('should use objects with defined toJSON method without rebuilding', function () {
+      const objProp = {};
+      const obj= {objProp, toJSON(){
+        return {ok: 1}
+      }};
+      const source = {x:1, y:2, obj};
+
+      const jsonObject = utils.toJSONObject(source);
+
+      assert.strictEqual(jsonObject.obj.objProp, objProp);
+      assert.strictEqual(JSON.stringify(jsonObject), JSON.stringify({x: 1, y:2, obj: {ok: 1}}))
+    });
+  });
 });