Fix usage of inherited Sinatra::Base classes keyword arguments

Using keyword arguments on inherited classes from Sinatra::Base breaks
with ArgumentError: wrong number of arguments (given X, expected 0).

This commit fixes the usage of it on Ruby3 by allowing the method to
accept both positional and keyword arguments.

.travis.yml

@@ -20,6 +20,7 @@ rvm:
   - 2.5.8
   - 2.6.6
   - 2.7.1
+  - 3.0.0-preview1
   - jruby-9.2.12.0
 
 script: ./.travis.sh
@@ -28,6 +29,7 @@ matrix:
   allow_failures:
     - rvm: 2.6.6
     - rvm: jruby-9.2.12.0
+    - rvm: 3.0.0-preview1
 
 notifications:
   slack:

lib/sinatra/base.rb

@@ -1522,8 +1522,8 @@ def prototype
       # 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, &bk)
-        instance = new!(*args, &bk)
+      def new(*args, **kwargs, &bk)
+        instance = kwargs.length > 0 ? new!(**kwargs, &bk) : new!(*args, &bk)
         Wrapper.new(build(instance).to_app, instance)
       end