Add spec for Struct#keyword_init?

core/struct/keyword_init_spec.rb

@@ -0,0 +1,21 @@
+require_relative '../../spec_helper'
+
+ruby_version_is "3.1" do
+  # See https://bugs.ruby-lang.org/issues/18008
+  describe "StructClass#keyword_init?" do
+    it "returns true for a struct that accepts keyword arguments to initialize" do
+      struct = Struct.new(:arg, keyword_init: true)
+      struct.keyword_init?.should be_true
+    end
+
+    it "returns false for a struct that does not accept keyword arguments to initialize" do
+      struct = Struct.new(:arg, keyword_init: false)
+      struct.keyword_init?.should be_false
+    end
+
+    it "returns nil for a struct that did not explicitly specify keyword_init" do
+      struct = Struct.new(:arg)
+      struct.keyword_init?.should be_nil
+    end
+  end
+end