cython: Fix Node.to_bin() decode

Don't convert the string to UTF-8, just bytes.
Fixes #109.
diff --git a/cython/plist.pyx b/cython/plist.pyx
index b148687..f6696d6 100644
--- a/cython/plist.pyx
+++ b/cython/plist.pyx
@@ -116,7 +116,7 @@
         plist_to_bin(self._c_node, &out, &length)
 
         try:
-            return _from_string_and_size(out, length)
+            return bytes(out[:length])
         finally:
             if out != NULL:
                 libc.stdlib.free(out)