core: Add Host header to hooks event

Closes https://github.com/tus/tusd/issues/935

pkg/handler/unrouted_handler.go

@@ -98,6 +98,12 @@ type HookEvent struct {
 }
 
 func newHookEvent(info FileInfo, r *http.Request) HookEvent {
+	// The Host header field is not present in the header map, see https://pkg.go.dev/net/http#Request:
+	// > For incoming requests, the Host header is promoted to the
+	// > Request.Host field and removed from the Header map.
+	// That's why we add it back manually.
+	r.Header.Set("Host", r.Host)
+
 	return HookEvent{
 		Upload: info,
 		HTTPRequest: HTTPRequest{