Fix failing tests after Stripe provider change

src/tests/api/test_orders.py

@@ -222,7 +222,7 @@ TEST_REFUNDS_RES = [
         "execution_date": "2017-12-01T10:00:00Z",
         "comment": None,
         "provider": "stripe",
-        "details": {},
+        "details": {"id": None},
         "state": "done",
         "amount": "23.00"
     },

src/tests/plugins/stripe/test_provider.py

@@ -85,6 +85,13 @@ class MockedCharge():
     id = 'ch_123345345'
     refunds = MockedRefunds()
 
+    def __str__(self):
+        return json.dumps({
+            'id': self.id,
+            'status': self.status,
+            'paid': self.paid,
+        })
+
     def refresh(self):
         pass