Fix TS definition for AxiosRequestTransformer

[brianhelba]

The callback AxiosRequestTransformer can always expect its headers
parameter to be defined. This is ensured immediately before the callback
is made.
See:

axios/lib/core/dispatchRequest.js

Lines 32 to 40 in 1025d12

	config.headers = config.headers || {};
	// Transform request data
	config.data = transformData.call(
	config,
	config.data,
	config.headers,
	config.transformRequest
	);

[jasonsaayman]

Thanks for the contribution