Fixing an issue that type 'null' is not assignable to validateStatus (a…

…xios#2773)

Co-authored-by: Xianming Zhong <chinesedfan@qq.com>
Co-authored-by: Jay <jasonsaayman@gmail.com>

index.d.ts

@@ -62,8 +62,8 @@ export interface AxiosRequestConfig {
   onUploadProgress?: (progressEvent: ProgressEvent) => void;
   onDownloadProgress?: (progressEvent: ProgressEvent) => void;
   maxContentLength?: number;
+  validateStatus?: ((status: number) => boolean | null);
   maxBodyLength?: number;
-  validateStatus?: (status: number) => boolean;
   maxRedirects?: number;
   socketPath?: string | null;
   httpAgent?: any;