The headers set with this option is only ever used in requests sent to a proxy - when there's also a request sent to a host.
The first line in a request (containing the method, usually a GET or POST) is NOT a header and cannot be replaced using this option. Only the lines following the request-line are headers. Adding this method line in this list of headers will only cause your request to send an invalid header.
Nil to this to reset back to no custom headers.