add MIME type of ico file

The server returns 500 error when requested '/favion.ico'
The handler worked fine, but send_file has failed with
- Missing MIME type for extension ".ico"
so I register mime type for .ico file

src/util/util.cr

@@ -27,6 +27,7 @@ def register_mime_types
     ".rar" => "application/x-rar-compressed",
     ".cbz" => "application/vnd.comicbook+zip",
     ".cbr" => "application/vnd.comicbook-rar",
+    ".ico" => "image/x-icon",
   }.each do |k, v|
     MIME.register k, v
   end