session_storage: Correctly handle string/byte types

In session_storage.py, store_data() stores data as the bytes data
type but get_data() is returning a string. Have get_data() return
bytes as well.

https://pagure.io/freeipa/issue/4985

Reviewed-By: Martin Basti <mbasti@redhat.com>

ipalib/rpc.py

@@ -887,7 +887,10 @@ class RPCClient(Connectible):
         # (possibly with more than one cookie).
         try:
             cookie_string = read_persistent_client_session_data(principal)
-        except Exception:
+            cookie_string = cookie_string.decode('utf-8')
+        except Exception as e:
+            self.log.debug('Error reading client session data: {err}'
+                           .format(err=e))
             return None
 
         # Search for the session cookie within the cookie string