core::io::writer : add get_type() method

The get_type() method can hint to users what kind of item might be
   under the hood.

src/libstd/net_tcp.rs

@@ -840,6 +840,9 @@ impl tcp_socket_buf of io::writer for @tcp_socket_buf {
     fn flush() -> int {
         0
     }
+    fn get_type() -> io::writer_type {
+        io::file
+    }
 }
 
 // INTERNAL API