net: ensure that ResolveIPAddr(addr.String()) reproduces addr

Updates #6465.

Change-Id: I5babbcf4c92dae47c823a41628b01e4ceb2332eb
Reviewed-on: https://go-review.googlesource.com/11951
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/ipraw_test.go

@@ -38,6 +38,8 @@ var resolveIPAddrTests = []resolveIPAddrTest{
 	{"", "127.0.0.1", &IPAddr{IP: IPv4(127, 0, 0, 1)}, nil}, // Go 1.0 behavior
 	{"", "::1", &IPAddr{IP: ParseIP("::1")}, nil},           // Go 1.0 behavior
 
+	{"ip4:icmp", "", &IPAddr{}, nil},
+
 	{"l2tp", "127.0.0.1", nil, UnknownNetworkError("l2tp")},
 	{"l2tp:gre", "127.0.0.1", nil, UnknownNetworkError("l2tp:gre")},
 	{"tcp", "1.2.3.4:123", nil, UnknownNetworkError("tcp")},

src/net/iprawsock.go

@@ -17,10 +17,11 @@ func (a *IPAddr) String() string {
 	if a == nil {
 		return "<nil>"
 	}
+	ip := ipEmptyString(a.IP)
 	if a.Zone != "" {
-		return a.IP.String() + "%" + a.Zone
+		return ip + "%" + a.Zone
 	}
-	return a.IP.String()
+	return ip
 }
 
 func (a *IPAddr) isWildcard() bool {