Add test for UdpSocket peer_addr

src/libstd/net/udp.rs

@@ -903,6 +903,16 @@ mod tests {
         })
     }
 
+    #[test]
+    fn socket_peer_ip4() {
+        each_ip(&mut |addr1, addr2| {
+            let server = t!(UdpSocket::bind(&addr1));
+            assert_eq!(server.peer_addr().unwrap_err().kind(), ErrorKind::NotConnected);
+            t!(server.connect(&addr2));
+            assert_eq!(addr2, t!(server.peer_addr()));
+        })
+    }
+
     #[test]
     fn udp_clone_smoke() {
         each_ip(&mut |addr1, addr2| {