ntop: Measure address size in bytes

`IPAddr.ntop` takes a binary representation of IP address, whose
length should be 4 or 16 *bytes* (not characters/codepoints).

This patch fixes the bug that `IPAddr.ntop` doesn't handle Strings in
a multibyte encoding correctly.

Fixes: ruby#56

lib/ipaddr.rb

@@ -111,7 +111,7 @@ def self.new_ntoh(addr)
   # Convert a network byte ordered string form of an IP address into
   # human readable form.
   def self.ntop(addr)
-    case addr.size
+    case addr.bytesize
     when 4
       addr.unpack('C4').join('.')
     when 16

test/test_ipaddr.rb

@@ -134,9 +134,15 @@ def test_s_new_ntoh
   def test_ntop
     # IPv4
     assert_equal("192.168.1.1", IPAddr.ntop("\xC0\xA8\x01\x01"))
+    assert_equal("192.168.1.1", IPAddr.ntop("\xC0\xA8\x01\x01".b))
+    assert_equal("10.231.140.171", IPAddr.ntop("\x0A\xE7\x8C\xAB"))  # Looks like 2-codepoint string in UTF-8
     # IPv6
     assert_equal("0000:0000:0000:0000:0000:0000:0000:0001",
                  IPAddr.ntop("\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01"))
+    assert_equal("0000:0000:0000:0000:0000:0000:0000:0001",
+                 IPAddr.ntop("\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01".b))
+    assert_equal("fe80:0000:0000:0000:f09f:9985:f09f:9986",
+                 IPAddr.ntop("\xFE\x80\x00\x00\x00\x00\x00\x00\xF0\x9F\x99\x85\xF0\x9F\x99\x86"))  # 10 codepoints in UTF-8
 
     # Invalid parameters
     assert_raise(IPAddr::AddressFamilyError) {
@@ -146,6 +152,10 @@ def test_ntop
     assert_raise(IPAddr::AddressFamilyError) {
       IPAddr.ntop("\xC0\xA8\x01\xFF1")
     }
+
+    assert_raise(IPAddr::AddressFamilyError) {
+      IPAddr.ntop("\x0Aあいう")  # Four coudepoints in UTF-8
+    }
   end
 
   def test_ipv4_compat