Fix keyword argument separation warnings on Ruby 2.7+

Do so in a way that is also compatible with previous versions.

Author: jeremyevans

lib/forwardable.rb

@@ -183,7 +183,9 @@ def def_instance_delegator(accessor, method, ali = method)
     gen = Forwardable._delegator_method(self, accessor, method, ali)
 
     # If it's not a class or module, it's an instance
-    (Module === self ? self : singleton_class).module_eval(&gen)
+    mod = Module === self ? self : singleton_class
+    mod.module_eval(&gen)
+    mod.send(:ruby2_keywords, ali) if RUBY_VERSION >= '2.7'
   end
 
   alias delegate instance_delegate
@@ -301,6 +303,7 @@ def def_single_delegator(accessor, method, ali = method)
     gen = Forwardable._delegator_method(self, accessor, method, ali)
 
     instance_eval(&gen)
+    singleton_class.send(:ruby2_keywords, ali) if RUBY_VERSION >= '2.7'
   end
 
   alias delegate single_delegate

test/test_forwardable.rb

@@ -16,6 +16,10 @@ def delegated1
     def delegated2
       RETURNED2
     end
+
+    def delegated1_kw(**kw)
+      [RETURNED1, kw]
+    end
   end
 
   def test_def_instance_delegator
@@ -38,6 +42,18 @@ def test_def_instance_delegator_constant
     end
   end
 
+  def test_def_instance_delegator_kw
+    %i[def_delegator def_instance_delegator].each do |m|
+      cls = forwardable_class do
+        __send__ m, :@receiver, :delegated1_kw
+      end
+
+      ary = cls.new.delegated1_kw b: 1
+      assert_same RETURNED1, ary[0]
+      assert_equal({b: 1}, ary[1])
+    end
+  end
+
   def test_def_instance_delegator_using_args_method_as_receiver
     %i[def_delegator def_instance_delegator].each do |m|
       cls = forwardable_class(