fix: IPROTO_ID auth method encoding

If encoding=None is used (or similar decoder configuration),
authentication method received in IPROTO_ID response is not an str, but
a bytes object, which is unexpected. This patch fixed the behavior.

Follows #269

Author: DifferentialOrange

Diff for: tarantool/response.py

@@ -30,6 +30,7 @@
     SchemaReloadException,
     tnt_strerror
 )
+from tarantool.schema import to_unicode
 
 from tarantool.msgpack_ext.unpacker import ext_hook as unpacker_ext_hook
 
@@ -397,4 +398,4 @@ def auth_type(self):
 
         if self._return_code != 0:
             return None
-        return self._body.get(IPROTO_AUTH_TYPE)
+        return to_unicode(self._body.get(IPROTO_AUTH_TYPE))