Freeze some constants to improve Ractor compatibility

[osyoyu]

Freeze Net::HTTP::SSL_IVNAMES, Net::HTTPResponse::CODE_CLASS_TO_OBJ and Net::HTTPResponse::CODE_TO_OBJ to improve Ractor compatibility.

This change allows the following code to work:

Ractor.new {
  uri = URI.parse('http://example.com')
  http = Net::HTTP.new(uri.host, uri.port)
  http.open_timeout = nil
  http.read_timeout = nil
  http.get('/index.html')
}

---

Possibilities are that this patch breaks code which modify these constants. I have quickly skimmed over GitHub Code Search and have found only one instance where CODE_TO_OBJ is mutated, from 17-year-old code:
https://github.com/tolsen/rubydav/blob/62776abc293073da894b08c22d76952c918525c6/lib/rubydav/webdav.rb#L267

rubydav already needs a number of modifications to run under Ruby 3.4.1 even without this patch, so I think this change won't introduce additional major damage.

[marshall-lee]

@hsbt Hi! Any chances this one is merged? Making HTTP requests in Ruby ractors become more & more desirable. And it seems like a safe change anyway. This sure will help a lot. Thanks