The request or target URL the header fields are to be retrieved from, unless overridden by init
Optionalinit: RequestInit
The request options the header fields are to be retrieved from, taking precedence over input
The header fields stated by init or by input, empty if neither states any
Retrieves the header fields of a request.
Header fields stated by the options replace those stated by a request input rather than adding to them, as
fetchhas it, so what is reported is the field list the origin server is asked with.