Handle case where ruby2_keywords method splats to ruby2_keywords method

Previously, the keyword hash was duped (which results in a regular
hash), but the dup was not marked as a keyword hash, causing the
hash not to be marked as keyword hash even though it should be.

test/ruby/test_keyword.rb

@@ -2649,6 +2649,14 @@ class TestKeywordArguments < Test::Unit::TestCase
         baz(*args)
       end
 
+      ruby2_keywords def foo_foo_bar(meth, *args)
+        foo_bar(meth, *args)
+      end
+
+      ruby2_keywords def foo_foo_baz(meth, *args)
+        foo_baz(meth, *args)
+      end
+
       ruby2_keywords def foo_mod(meth, *args)
         args << 1
         send(meth, *args)
@@ -2761,6 +2769,12 @@ class TestKeywordArguments < Test::Unit::TestCase
     assert_equal([1, h1], o.store_foo(:baz, 1, :a=>1))
     assert_equal([[1], h1], o.foo_bar(1, :a=>1))
     assert_equal([1, h1], o.foo_baz(1, :a=>1))
+    assert_equal([[1], h1], o.foo(:foo, :bar, 1, :a=>1))
+    assert_equal([1, h1], o.foo(:foo, :baz, 1, :a=>1))
+    assert_equal([[1], h1], o.foo(:foo_bar, 1, :a=>1))
+    assert_equal([1, h1], o.foo(:foo_baz, 1, :a=>1))
+    assert_equal([[1], h1], o.foo_foo_bar(1, :a=>1))
+    assert_equal([1, h1], o.foo_foo_baz(1, :a=>1))
 
     assert_equal([[1], h1], o.foo(:bar, 1, **h1))
     assert_equal([1, h1], o.foo(:baz, 1, **h1))
@@ -2770,6 +2784,12 @@ class TestKeywordArguments < Test::Unit::TestCase
     assert_equal([1, h1], o.store_foo(:baz, 1, **h1))
     assert_equal([[1], h1], o.foo_bar(1, **h1))
     assert_equal([1, h1], o.foo_baz(1, **h1))
+    assert_equal([[1], h1], o.foo(:foo, :bar, 1, **h1))
+    assert_equal([1, h1], o.foo(:foo, :baz, 1, **h1))
+    assert_equal([[1], h1], o.foo(:foo_bar, 1, **h1))
+    assert_equal([1, h1], o.foo(:foo_baz, 1, **h1))
+    assert_equal([[1], h1], o.foo_foo_bar(1, **h1))
+    assert_equal([1, h1], o.foo_foo_baz(1, **h1))
 
     assert_equal([[h1], {}], o.foo(:bar, h1, **{}))
     assert_equal([h1], o.foo(:baz, h1, **{}))
@@ -2779,6 +2799,12 @@ class TestKeywordArguments < Test::Unit::TestCase
     assert_equal([h1], o.store_foo(:baz, h1, **{}))
     assert_equal([[h1], {}], o.foo_bar(h1, **{}))
     assert_equal([h1], o.foo_baz(h1, **{}))
+    assert_equal([[h1], {}], o.foo(:foo, :bar, h1, **{}))
+    assert_equal([h1], o.foo(:foo, :baz, h1, **{}))
+    assert_equal([[h1], {}], o.foo(:foo_bar, h1, **{}))
+    assert_equal([h1], o.foo(:foo_baz, h1, **{}))
+    assert_equal([[h1], {}], o.foo_foo_bar(h1, **{}))
+    assert_equal([h1], o.foo_foo_baz(h1, **{}))
 
     assert_warn(/The last argument is used as the keyword parameter.* for `bar'/m) do
       assert_equal([[1], h1], o.foo(:bar, 1, h1))

vm_args.c

@@ -740,6 +740,9 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
                 rest_last = rb_hash_dup(rest_last);
                 RARRAY_ASET(args->rest, len - 1, rest_last);
                 kw_flag |= VM_CALL_KW_SPLAT;
+                if (iseq->body->param.flags.ruby2_keywords) {
+                    remove_empty_keyword_hash = 0;
+                }
             }
             else {
                 rest_last = 0;