Fix keyword argument separation warning in method_missing

vm_call_method_missing was dropping VM_CALL_KW_SPLAT, so this just
makes it not drop it, to get the same behavior as calling the method
directly.

test/ruby/test_keyword.rb

@@ -443,6 +443,79 @@ class TestKeywordArguments < Test::Unit::TestCase
     assert_equal([1, h3], c.send(:m, **h3))
   end
 
+  def test_send_kwsplat
+    kw = {}
+    h = {'a'=>1}
+    h2 = {'a'=>1}
+    h3 = {'a'=>1, :a=>1}
+
+    c = Object.new
+    def c.method_missing(_, *args)
+      args
+    end
+    assert_equal([], c.send(:m, **{}))
+    assert_equal([], c.send(:m, **kw))
+    assert_equal([h], c.send(:m, **h))
+    assert_equal([h2], c.send(:m, **h2))
+    assert_equal([h3], c.send(:m, **h3))
+
+    c.singleton_class.remove_method(:method_missing)
+    def c.method_missing(_); end
+    assert_nil(c.send(:m, **{}))
+    assert_nil(c.send(:m, **kw))
+    assert_raise(ArgumentError) { c.send(:m, **h) }
+    assert_raise(ArgumentError) { c.send(:m, **h2) }
+    assert_raise(ArgumentError) { c.send(:m, **h3) }
+
+    c.singleton_class.remove_method(:method_missing)
+    def c.method_missing(_, args)
+      args
+    end
+    assert_raise(ArgumentError) { c.send(:m, **{}) }
+    assert_raise(ArgumentError) { c.send(:m, **kw) }
+    assert_equal(h, c.send(:m, **h))
+    assert_equal(h2, c.send(:m, **h2))
+    assert_equal(h3, c.send(:m, **h3))
+
+    c.singleton_class.remove_method(:method_missing)
+    def c.method_missing(_, **args)
+      args
+    end
+    assert_equal(kw, c.send(:m, **{}))
+    assert_equal(kw, c.send(:m, **kw))
+    assert_equal(h, c.send(:m, **h))
+    assert_equal(h2, c.send(:m, **h2))
+    assert_equal(h3, c.send(:m, **h3))
+
+    c.singleton_class.remove_method(:method_missing)
+    def c.method_missing(_, arg, **args)
+      [arg, args]
+    end
+    assert_raise(ArgumentError) { c.send(:m, **{}) }
+    assert_warn(/The keyword argument is passed as the last hash parameter.* for `method_missing'/m) do
+      assert_equal([kw, kw], c.send(:m, **kw))
+    end
+    assert_warn(/The keyword argument is passed as the last hash parameter.* for `method_missing'/m) do
+      assert_equal([h, kw], c.send(:m, **h))
+    end
+    assert_warn(/The keyword argument is passed as the last hash parameter.* for `method_missing'/m) do
+      assert_equal([h2, kw], c.send(:m, **h2))
+    end
+    assert_warn(/The keyword argument is passed as the last hash parameter.* for `method_missing'/m) do
+      assert_equal([h3, kw], c.send(:m, **h3))
+    end
+
+    c.singleton_class.remove_method(:method_missing)
+    def c.method_missing(_, arg=1, **args)
+      [arg=1, args]
+    end
+    assert_equal([1, kw], c.send(:m, **{}))
+    assert_equal([1, kw], c.send(:m, **kw))
+    assert_equal([1, h], c.send(:m, **h))
+    assert_equal([1, h2], c.send(:m, **h2))
+    assert_equal([1, h3], c.send(:m, **h3))
+  end
+
   def p1
     Proc.new do |str: "foo", num: 424242|
       [str, num]

vm_insnhelper.c

@@ -2408,6 +2408,9 @@ vm_call_method_missing(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp,
     argc = calling->argc+1;
 
     ci_entry.flag = VM_CALL_FCALL | VM_CALL_OPT_SEND;
+    if (orig_ci->flag & VM_CALL_KW_SPLAT) {
+        ci_entry.flag |= VM_CALL_KW_SPLAT;
+    }
     ci_entry.mid = idMethodMissing;
     ci_entry.orig_argc = argc;
     ci = &ci_entry;