Never use Bytes without any 'Adapter'

Otherwise we have binary/bytes as values inside the dict, rather than a
hexadecimal string.  That's ugly when printing without json formatting.

Change-Id: Ia3e7c4791d11bd4e3719a43d58e11e05ec986d1f
diff --git a/pySim/gsm_r.py b/pySim/gsm_r.py
index 3364b3d..b48469e 100644
--- a/pySim/gsm_r.py
+++ b/pySim/gsm_r.py
@@ -185,7 +185,7 @@
         super().__init__(fid='6ff4', sfid=None,
                          name='EF.Shunting', desc='Shunting', size={8, 8})
         self._construct = Struct('common_gid'/Int8ub,
-                                 'shunting_gid'/Bytes(7))
+                                 'shunting_gid'/HexAdapter(Bytes(7)))
 
 
 class EF_GsmrPLMN(LinFixedEF):