Directory Listing: Add UTF-8 Charset to the response Content type

Within the directory listing handler, the absence of an explicit charset in the content type has led to issues, particularly with directories containing non-ASCII symbols, resulting in broken displays.

Adding charset charset=UTF-8 to the Content-type header fixes this issue.

src/kemal/static_file_handler.cr

@@ -57,7 +57,7 @@ module Kemal
           end
           send_file(context, file_path)
         elsif config.is_a?(Hash) && config.fetch("dir_listing", false)
-          context.response.content_type = "text/html"
+          context.response.content_type = "text/html;charset=UTF-8;"
           directory_listing(context.response, request_path, file_path)
         else
           call_next(context)