Ruby 3.4 produces frozen string warnings

Calling `.to_s` on symbol will return frozen string in future
https://github.com/sparklemotion/mechanize/blob/main/lib/mechanize/form.rb#L259

```ruby
:test.to_s.chomp!
DEPRECATION WARNING: warning: string returned by :test.to_s will be frozen in the future
 (called from String#chomp!)
```