Fix encoding bug in tests on Ruby 3

[apainintheneck]

We need to guess the HTML encoding here otherwise some tests fail.

Failures:

  1) Readability images should show one image, but outside of the best candidate
     Failure/Error: @input = @input.gsub(REGEXES[:replaceBrsRe], '</p><p>')

     ArgumentError:
       invalid byte sequence in UTF-8
     # ./lib/readability.rb:51:in `gsub'
     # ./lib/readability.rb:51:in `initialize'
     # ./spec/readability_spec.rb:80:in `new'
     # ./spec/readability_spec.rb:80:in `block (3 levels) in <top (required)>'

  2) Readability the cant_read.html fixture should work on the cant_read.html fixture with some allowed tags
     Failure/Error: @input = @input.gsub(REGEXES[:replaceBrsRe], '</p><p>')

     ArgumentError:
       invalid byte sequence in UTF-8
     # ./lib/readability.rb:51:in `gsub'
     # ./lib/readability.rb:51:in `initialize'
     # ./spec/readability_spec.rb:555:in `new'
     # ./spec/readability_spec.rb:555:in `block (3 levels) in <top (required)>'

Fixes #87

It also adds the latest Ruby 3 version to CI to test for these sort of bugs regularly.

[cantino]

Thanks!