Fix warnings

[jeffschoner]

Fixes a couple of warnings

• Adds the base64 gem to the gemspec. Starting in Ruby 3.4.0, this will be required since it's moving out of the standard library. For 3.3.0, this is showing up as a warning. See https://www.ruby-lang.org/en/news/2023/12/25/ruby-3-3-0-released/ for more information
• Assertions that specs do not raise a specific error have been made more general to assert that a test raises no error at all. This removes a warning and should make these tests slightly more robust