fix: allow multiple informational status (#961)

Informational status in the range 100-199 may be sent multiple times, unlike statuses in the range 200-599.

This follows usage in the stdlib:
https://github.com/golang/go/blob/485ed2fa5b5e0b7067ef72a0f4bdc9ca12b77ed7/src/net/http/server.go#L1216

middleware/wrap_writer.go

@@ -81,7 +81,11 @@ type basicWriter struct {
 }
 
 func (b *basicWriter) WriteHeader(code int) {
-	if !b.wroteHeader {
+	if code >= 100 && code <= 199 && code != http.StatusSwitchingProtocols {
+		if !b.discard {
+			b.ResponseWriter.WriteHeader(code)
+		}
+	} else if !b.wroteHeader {
 		b.code = code
 		b.wroteHeader = true
 		if !b.discard {