[FIX] payment_buckaroo: copy values dict to avoid altering original dict

_buckaroo_generate_digital_sign uses the values dict to generate the shasign
At some point, it alters the dict, it removes BRQ_SIGNATURE, but, as the values dict was not copied, it altered the original dict.
So, the key BRQ_SIGNATURE was not anymore present for methods called after _buckaroo_generate_digital_sign.
For instance, the overriden method form_feedback of website_sale payment

addons/payment_buckaroo/models/buckaroo.py

@@ -57,6 +57,8 @@ def get_value(key):
                 return values[key]
             return ''
 
+        values = dict(values or {})
+
         if inout == 'out':
             if 'BRQ_SIGNATURE' in values:
                 del values['BRQ_SIGNATURE']