Fix shared example breaking on Ruby 3.1

On Ruby 3.1, a proc with |objects, type: :x| called with a single
array argument destructures it — so objects receives the first
element instead of the full array. Ruby 3.2+ changed this behavior,
which is why the test was passing in CI (Ruby 3.3) but failed once
we pinned the local version to the minimum required Ruby (3.1.0).

Using a splat (*objects) avoids the destructuring entirely: it
collects all positional arguments into an array, so objects.first
is always the classes array regardless of Ruby version.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: thiagoa

lib/duck_typer/rspec.rb

@@ -19,7 +19,8 @@
 module DuckTyper
   module RSpec
     def self.define_shared_example(name = "an interface")
-      ::RSpec.shared_examples name do |objects, type: :instance_methods, methods: nil|
+      ::RSpec.shared_examples name do |*objects, type: :instance_methods, methods: nil|
+        objects = objects.first
         # We intentionally avoid reusing the have_matching_interfaces matcher
         # here. Since this shared example is defined in gem code, RSpec filters
         # it from its backtrace, causing the Failure/Error: line to show an