[FIX] website_payment: stop treating _registration_render result as a list

I assume this was necessary  before the migration of payment because
an api.one wrapper was added by the auto-guess api feature.

Since payment has been migrated, this decorator is no longer present
and the result of _registration_render is no longer encapsulated in
a list. Therefore, taking the zeroth element was akin to taking only
the first character of the rendered html. Usually \n, which is not
useful but which also prevented the crash to be easily visible.

addons/website_payment/controllers/main.py

@@ -16,7 +16,7 @@ class website_payment(http.Controller):
             'acquirers': acquirers
         }
         for acquirer in acquirers:
-            acquirer.form = acquirer.sudo()._registration_render(request.env.user.partner_id.id, {'error': {}, 'error_message': [], 'return_url': '/my/payment_method', 'json': False, 'bootstrap_formatting': True})[0]
+            acquirer.form = acquirer.sudo()._registration_render(request.env.user.partner_id.id, {'error': {}, 'error_message': [], 'return_url': '/my/payment_method', 'json': False, 'bootstrap_formatting': True})
         return request.website.render("website_payment.pay_methods", values)
 
     @http.route(['/website_payment/delete/'], methods=['POST'], type='http', auth="user", website=True)