[FIX] payment_paypal: avoid json decode error

with post.pop('custom', '{}'), if the key 'custom' is not there, it will return '{}', but if the key is there, but the value is equivalent to False, the json decode will fail.

addons/payment_paypal/controllers/main.py

@@ -24,7 +24,7 @@ class PaypalController(http.Controller):
         """ Extract the return URL from the data coming from paypal. """
         return_url = post.pop('return_url', '')
         if not return_url:
-            custom = json.loads(post.pop('custom', '{}'))
+            custom = json.loads(post.pop('custom', False) or '{}')
             return_url = custom.get('return_url', '/')
         return return_url