ca: Decode header before providing it to json.loads .

Consistently with how doBackup encodes the result of json.dumps .

caucase/ca.py

@@ -894,7 +894,7 @@ class UserCertificateAuthority(CertificateAuthority):
       '<I',
       read(struct.calcsize('<I')),
     )
-    header = json.loads(read(header_len))
+    header = json.loads(utils.toUnicode(read(header_len)))
     symetric_cipher_name = header['cipher']['name']
     try:
       HMAC_PAYLOAD_SIZE = cls.__backup_format_dict[symetric_cipher_name]