erp5_stripe: Fix test simulating a webhook request made by stripe.

The real request has the `CONTENT_TYPE` header so do the same in the test. This
was working anyway without the header on py2 but not on py3.

bt5/erp5_stripe/TestTemplateItem/portal_components/test.erp5.testStripe.py

@@ -477,6 +477,7 @@ class TestStripePaymentSession(ERP5TypeTestCase):
           })
         }).encode()),
         request_method="POST",
+        env={'CONTENT_TYPE': 'application/json'},
         handle_errors=False)
       self.assertEqual(200, ret.getStatus())
       self.tic()
@@ -673,6 +674,7 @@ class TestStripePaymentSession(ERP5TypeTestCase):
           })
         }).encode()),
         request_method="POST",
+        env={'CONTENT_TYPE': 'application/json'},
         handle_errors=False)
       self.assertEqual(200, ret.getStatus())
       self.tic()