This function lets the application iterate over all available headers from within the given origins that arrived in the request.
The request argument tells libcurl from which request you want headers from.
If previous is set to NULL, this function returns a pointer to the first header. The application can then use that pointer as an argument to the next call to iterate over all available headers within the same origin and request context.
When this function returns NULL, there are no more headers within the context.
See Header struct for details on the curl_header struct that function this returns a pointer to.