Fix TS definition for AxiosRequestTransformer (#4201)

The callback `AxiosRequestTransformer` can always expect its `headers`
parameter to be defined. This is ensured immediately before the callback
is made.
See: https://github.com/axios/axios/blob/1025d1231a7747503188459dd5a6d1effdcea928/lib/core/dispatchRequest.js#L32-L40

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

index.d.ts

@@ -6,7 +6,7 @@ export type AxiosResponseHeaders = Record<string, string> & {
 };
 
 export interface AxiosRequestTransformer {
-  (data: any, headers?: AxiosRequestHeaders): any;
+  (data: any, headers: AxiosRequestHeaders): any;
 }
 
 export interface AxiosResponseTransformer {