Test multiple arguments in Set#merge

References:
* https://rubyreferences.github.io/rubychanges/3.3.html#setmerge-accepts-multiple-arguments
* https://docs.ruby-lang.org/en/3.3/Set.html#method-i-merge

library/set/merge_spec.rb

@@ -16,4 +16,16 @@
     -> { Set[1, 2].merge(1) }.should raise_error(ArgumentError)
     -> { Set[1, 2].merge(Object.new) }.should raise_error(ArgumentError)
   end
+
+  ruby_version_is ""..."3.3" do
+    it "accepts only a single argument" do
+      -> { Set[].merge([], []) }.should raise_error(ArgumentError, "wrong number of arguments (given 2, expected 1)")
+    end
+  end
+
+  ruby_version_is "3.3" do
+    it "accepts multiple arguments" do
+      Set[:a, :b].merge(Set[:b, :c], [:d]).should == Set[:a, :b, :c, :d]
+    end
+  end
 end