add a test.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44666 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/socket/test_sockopt.rb

@@ -43,6 +43,13 @@ class TestSocketOption < Test::Unit::TestCase
     assert_equal(128, sockopt.ipv4_multicast_ttl)
   end
 
+  def test_ipv4_multicast_ttl_size
+    expected_size = Socket.open(:INET, :DGRAM) {|s|
+      s.getsockopt(:IP, :MULTICAST_TTL).to_s.bytesize
+    }
+    assert_equal(expected_size, Socket::Option.ipv4_multicast_ttl(0).to_s.bytesize)
+  end
+
   def test_unpack
     sockopt = Socket::Option.new(:INET, :SOCKET, :KEEPALIVE, [1].pack("i"))
     assert_equal([1], sockopt.unpack("i"))