allow for blank ip addresses (spf13#316)

Author: duhruh

ip.go

@@ -16,6 +16,9 @@ func newIPValue(val net.IP, p *net.IP) *ipValue {
 
 func (i *ipValue) String() string { return net.IP(*i).String() }
 func (i *ipValue) Set(s string) error {
+	if s == "" {
+		return nil
+	}
 	ip := net.ParseIP(strings.TrimSpace(s))
 	if ip == nil {
 		return fmt.Errorf("failed to parse IP: %q", s)

ip_test.go

@@ -24,7 +24,7 @@ func TestIP(t *testing.T) {
 		{"1.2.3.4", true, "1.2.3.4"},
 		{"127.0.0.1", true, "127.0.0.1"},
 		{"255.255.255.255", true, "255.255.255.255"},
-		{"", false, ""},
+		{"", true, "0.0.0.0"},
 		{"0", false, ""},
 		{"localhost", false, ""},
 		{"0.0.0", false, ""},