avoid duplication of .gz suffix

prevents an issue where when a .gz file is requested it attempts to append an additional .gz suffix prior to checking if the file exists, i.e. the extension becomes .gz.gz

Author: GhostCore07

lib/core/index.js

@@ -201,7 +201,7 @@ module.exports = function createMiddleware(_dir, _options) {
       )
     );
     // determine compressed forms if they were to exist
-    gzippedFile = `${file}.gz`;
+    gzippedFile = file.slice(-3) === ".gz" ? file : `${file}.gz`;
     brotliFile = `${file}.br`;
 
     Object.keys(headers).forEach((key) => {