specs: puma 6 compatibility

[pcai]

Bug report

Current behavior:
specs fail with puma 6

Steps to reproduce current behavior:

bundle exec rspec in master:

...
       Failure/Error: Puma::Events.new($stdout, $stderr)

       ArgumentError:
         wrong number of arguments (given 2, expected 0)
       # ./vendor/bundle/ruby/2.7.0/gems/puma-6.0.0/lib/puma/events.rb:11:in `initialize'
       # ./spec/integration/support/server.rb:62:in `new'
       # ./spec/integration/support/server.rb:62:in `events'
       # ./spec/integration/support/server.rb:31:in `initialize'
       # ./spec/integration/support/server.rb:10:in `new'
       # ./spec/integration/support/server.rb:10:in `run'
       # ./spec/savon/options_spec.rb:11:in `block (2 levels) in <top (required)>'

Expected behavior:

tests pass

System information:

• ruby version: all
• savon version: 2.13.1 // master