XEP-0231: do not crash if max-age is None

it is only RECOMMENDED

slixmpp/plugins/xep_0231/bob.py

@@ -89,6 +89,7 @@ class XEP_0231(BasePlugin):
         bob['data'] = data
         bob['type'] = mtype
         bob['cid'] = cid
+        if max_age is not None:
             bob['max_age'] = max_age
 
         await self.api['set_bob'](args=bob)

slixmpp/plugins/xep_0231/stanza.py

@@ -18,9 +18,13 @@ class BitsOfBinary(ElementBase):
     interfaces = {'cid', 'max_age', 'type', 'data'}
 
     def get_max_age(self):
+        try:
             return int(self._get_attr('max-age'))
+        except ValueError:
+            return None
 
     def set_max_age(self, value):
+        if value is not None:
             self._set_attr('max-age', str(value))
 
     def get_data(self):