Do not parse an empty string as an IpAddress.

This is more likely to result in downstream confusion than not.
Let downstream code decide exactly what it wants to do with
an empty string; be conservative here.

src/parsers.rs

@@ -127,9 +127,6 @@ impl<'a> Parser<'a> {
     }
 
     fn accept_ip(&mut self) -> Result<IpAddress> {
-        if let Some(()) = self.try(|p| p.accept_eof()) {
-            return Ok(IpAddress::Unspecified)
-        }
         if let Some(ipv4) = self.try(|p| p.accept_ipv4()) {
             return Ok(IpAddress::Ipv4(ipv4))
         }
@@ -158,7 +155,7 @@ impl FromStr for Ipv4Address {
 impl FromStr for IpAddress {
     type Err = ();
 
-    /// Parse a string representation of an IPv4 address.
+    /// Parse a string representation of an IP address.
     fn from_str(s: &str) -> Result<IpAddress> {
         Parser::new(s).until_eof(|p| p.accept_ip())
     }
@@ -229,8 +226,7 @@ mod test {
 
     #[test]
     fn test_ip() {
-        assert_eq!(IpAddress::from_str(""),
-                   Ok(IpAddress::Unspecified));
+        assert_eq!(IpAddress::from_str(""), Err(()));
         assert_eq!(IpAddress::from_str("1.2.3.4"),
                    Ok(IpAddress::Ipv4(Ipv4Address([1, 2, 3, 4]))));
         assert_eq!(IpAddress::from_str("x"), Err(()));