Fixing an issue that type 'null' and 'undefined' is not assignable to…

… validateStatus when typescript strict option is enabled

index.d.ts

@@ -62,7 +62,7 @@ export interface AxiosRequestConfig {
   onUploadProgress?: (progressEvent: ProgressEvent) => void;
   onDownloadProgress?: (progressEvent: ProgressEvent) => void;
   maxContentLength?: number;
-  validateStatus?: ((status: number) => boolean | null);
+  validateStatus?: ((status: number) => boolean) | null | undefined;
   maxBodyLength?: number;
   maxRedirects?: number;
   socketPath?: string | null;