Allow only one argument for keyword_init struct

```
irb(main):001:0> RUBY_VERSION
=> "2.6.5"
irb(main):002:0> S = Struct.new(:foo, keyword_init: true)
=> S(keyword_init: true)
irb(main):003:0> S.new({foo: 23424}, 234) # I don't think this is intentional
=> #<struct S foo=23424>
irb(main):004:0>
```

Tightening this up should inform users when they are confused about
whether a struct is `keyword_init`.

1 files changed, 1 insertions, 0 deletions

diff --git a/test/ruby/test_struct.rb b/test/ruby/test_struct.rb
index ec7728b197..22a6ce8241 100644
--- a/test/ruby/test_struct.rb
+++ b/test/ruby/test_struct.rb
@@ -105,6 +105,7 @@ module TestStruct
     @Struct.new("KeywordInitFalse", :a, :b, keyword_init: false)
 
     assert_raise(ArgumentError) { @Struct::KeywordInitTrue.new(1, 2) }
+    assert_raise(ArgumentError) { @Struct::KeywordInitTrue.new({a: 100}, 2) }
     assert_nothing_raised { @Struct::KeywordInitFalse.new(1, 2) }
     assert_nothing_raised { @Struct::KeywordInitTrue.new(a: 1, b: 2) }
     assert_raise(ArgumentError) { @Struct::KeywordInitTrue.new(1, b: 2) }