Copy compare_by_identity flag for empty hashes in Hash.ruby2_keywords_hash

This was already copied for non-empty hashes.  As Hash.ruby2_keywords_hash
copies default values, it should also copy the compare_by_identity flag.

Partially Fixes [Bug #19113]

hash.c

@@ -1972,9 +1972,12 @@ static VALUE
 rb_hash_s_ruby2_keywords_hash(VALUE dummy, VALUE hash)
 {
     Check_Type(hash, T_HASH);
-    hash = rb_hash_dup(hash);
-    RHASH(hash)->basic.flags |= RHASH_PASS_AS_KEYWORDS;
-    return hash;
+    VALUE tmp = rb_hash_dup(hash);
+    if (RHASH_EMPTY_P(hash) && rb_hash_compare_by_id_p(hash)) {
+        rb_hash_compare_by_id(tmp);
+    }
+    RHASH(tmp)->basic.flags |= RHASH_PASS_AS_KEYWORDS;
+    return tmp;
 }
 
 struct rehash_arg {

spec/ruby/core/hash/ruby2_keywords_hash_spec.rb

@@ -56,4 +56,28 @@ describe "Hash.ruby2_keywords_hash" do
   it "raises TypeError for non-Hash" do
     -> { Hash.ruby2_keywords_hash(nil) }.should raise_error(TypeError)
   end
+
+  it "retains the default value" do
+    hash = Hash.new(1)
+    Hash.ruby2_keywords_hash(hash).default.should == 1
+    hash[:a] = 1
+    Hash.ruby2_keywords_hash(hash).default.should == 1
+  end
+
+  it "retains the default_proc" do
+    pr = proc { |h, k| h[k] = [] }
+    hash = Hash.new(&pr)
+    Hash.ruby2_keywords_hash(hash).default_proc.should == pr
+    hash[:a] = 1
+    Hash.ruby2_keywords_hash(hash).default_proc.should == pr
+  end
+
+  ruby_version_is '3.3' do
+    it "retains compare_by_identity_flag" do
+      hash = {}.compare_by_identity
+      Hash.ruby2_keywords_hash(hash).compare_by_identity?.should == true
+      hash[:a] = 1
+      Hash.ruby2_keywords_hash(hash).compare_by_identity?.should == true
+    end
+  end
 end