* Fix ruby 3.2 spec
* Run CI on ruby 3.2

Add ruby 3.2 to list of supported versions

Ran into an issue making requests to a server I don't control (Microsoft
ASP.NET) where it was giving me errors POSTing JSON.

This header works: `Content-Type: application/json; charset=utf-8`
This did not: `Content-Type: application/json; charset=UTF-8`

HTTP.rb uses Ruby `String#encoding.name`, which returns "UTF" in all
caps, which is causing the `charset=UTF-8`.

[MDN says "Case insensitive, lowercase
preferred"](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Type),
so I just downcased the encoding name, and added a test.

This PR adds `base64` to runtime dependency to suppress the following Ruby 3.3's warning:

```console
$ ruby -wve 'require "http"'
ruby 3.3.0dev (2023-08-14T15:48:39Z master 52837fcec2) [x86_64-darwin22]
/Users/koic/.rbenv/versions/3.3.0-dev/lib/ruby/gems/3.3.0+0/gems/http-5.1.1/lib/http/chainable.rb:3:
warning: base64 which will be not part of the default gems since Ruby 3.4.0
```

cf: rails/rails#48907

Co-authored-by: Richard Peng <richard@withpersona.com>

* Prevent CRLF injection due to broken URL normalizer

* DRY

This method is used by several caching servers, like Squid, Varnish or Fastly.