[FIX] payment_paypal: accept GET requests for Paypal DPN

Although we have been reluctant to perform this change, a specific
use case can cause customers to be redirect to the Odoo DPN url
with a GET request.

This happens when a Paypal Merchant account has the feature Guest
Checkout active; in that case, a customer can pay without having
a Paypal account (using only his credit card) and will *not* be
subjected to auto-return; as detailed here:
https://www.sandbox.paypal.com/be/cgi-bin/webscr?cmd=p/pop/help-account-optional
Request coming from that payment flow will always trigger a GET
request, causing the customer to be welcomed by a
405 - Method Not allowed
error on the Odoo server. The payment is normally correctly processed
through IPN, so this does not normally causes loss of data; however
this is not a nice way to welcome back your customer right after
they pay you.

addons/payment_paypal/controllers/main.py

@@ -68,7 +68,7 @@ class PaypalController(http.Controller):
         self.paypal_validate_data(**post)
         return ''
 
-    @http.route('/payment/paypal/dpn', type='http', auth="none", methods=['POST'])
+    @http.route('/payment/paypal/dpn', type='http', auth="none", methods=['POST', 'GET'])
     def paypal_dpn(self, **post):
         """ Paypal DPN """
         _logger.info('Beginning Paypal DPN form_feedback with post data %s', pprint.pformat(post))  # debug