Use URI.open instead of open in specs to pass with Ruby3.0

[boutil]

In Debian, when tring to build sham_rack with ruby3.0, we noticed the spec failures described in
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=996386

They are caused by the fact that in spec/sham_rack_spec.rb' opens urls with openinstead ofURI.open`, which throws errors like

1) ShamRack mounted Rack application can be accessed using open-uri
     Failure/Error: response = open("http://www.greetings.com")

     Errno::ENOENT:
       No such file or directory @ rb_sysopen - http://www.greetings.com
     # ./spec/sham_rack_spec.rb:42:in `initialize'
     # ./spec/sham_rack_spec.rb:42:in `open'
     # ./spec/sham_rack_spec.rb:42:in `block (3 levels) in <top (required)>'

The following patch is aimed at replacing these open by URI.open.