fix(#50): user_agent method returns 'Option<&NgxStr>' and not '&NgxStr'

Author: mtb0x1

examples/curl.rs

@@ -111,7 +111,7 @@ http_request_handler!(curl_access_handler, |request: &mut http::Request| {
 
     match co.enable {
         true => {
-            if request.user_agent().as_bytes().starts_with(b"curl") {
+            if request.user_agent().is_some_and(|ua| ua.as_bytes().starts_with(b"curl")) {
                 http::HTTPStatus::FORBIDDEN.into()
             } else {
                 core::Status::NGX_DECLINED