Fix TS definition for AxiosRequestTransformer (#4201)
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 <4f5661b92532aa65dc137cefa1d321b17b131199@gmail.com>
axios/axios,axios/axios,axios/axios