Use ruby2_keywords for delegation

[dentarg]

Follows "The Only Correct Way" from https://eregon.me/blog/2021/02/13/correct-delegation-in-ruby-2-27-3.html

Related to #1670
Fixes #1749

[yaauie]

My minimal reproduction from #1670 passes with this patch "applied", and a Logstash that has had its plugin manager invoked resulting in Sinatra 2.2.0 being installed and run via jruby is also fixed by applying this patch:

diff --git a/vendor/bundle/jruby/2.5.0/gems/sinatra-2.2.0/lib/sinatra/base.rb b/vendor/bundle/jruby/2.5.0/gems/sinatra-2.2.0/lib/sinatra/base.rb
index 6ced0a6..986fe45 100644
--- a/vendor/bundle/jruby/2.5.0/gems/sinatra-2.2.0/lib/sinatra/base.rb
+++ b/vendor/bundle/jruby/2.5.0/gems/sinatra-2.2.0/lib/sinatra/base.rb
@@ -1533,10 +1533,11 @@ module Sinatra
       # Create a new instance of the class fronted by its middleware
       # pipeline. The object is guaranteed to respond to #call but may not be
       # an instance of the class new was called on.
-      def new(*args, **kwargs, &bk)
-        instance = new!(*args, **kwargs, &bk)
+      def new(*args, &bk)
+        instance = new!(*args, &bk)
         Wrapper.new(build(instance).to_app, instance)
       end
+      ruby2_keywords :new if respond_to?(:ruby2_keywords, true)
 
       # Creates a Rack::Builder instance with all the middleware set up and
       # the given +app+ as end point.

[kares]

Hey @jkowens any chance to push 2.2.1 to unbreak JRuby users (accidentally) installing latest sinatra?

[jkowens]

We'll have to check with @namusyaka to see about cutting a 2.2.1 release.