[FIX[ iap: encoding of Qweb render

The Qweb render method returns a strign encoded in byte.
As we want to dump this into a json string in iap.charge,
we need to decode it properly in UTF-8, before the dump.
This problem appeared in Python3.

addons/iap/models/iap.py

@@ -95,7 +95,7 @@ def charge(env, key, account_token, credit, description=None, credit_template=No
     except InsufficientCreditError as e:
         if credit_template:
             arguments = json.loads(e.args[0])
-            arguments['body'] = env['ir.qweb'].render(credit_template)
+            arguments['body'] = env['ir.qweb'].render(credit_template).decode("utf-8")
             e.args = (json.dumps(arguments),)
         raise e
     try: