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

it is only RECOMMENDED

slixmpp/plugins/xep_0231/bob.py

@@ -89,7 +89,8 @@ class XEP_0231(BasePlugin):
         bob['data'] = data
         bob['type'] = mtype
         bob['cid'] = cid
-        bob['max_age'] = max_age
+        if max_age is not None:
+            bob['max_age'] = max_age
 
         await self.api['set_bob'](args=bob)
         # Schedule destruction of the data

slixmpp/plugins/xep_0231/stanza.py

@@ -18,10 +18,14 @@ class BitsOfBinary(ElementBase):
     interfaces = {'cid', 'max_age', 'type', 'data'}
 
     def get_max_age(self):
-        return int(self._get_attr('max-age'))
+        try:
+            return int(self._get_attr('max-age'))
+        except ValueError:
+            return None
 
     def set_max_age(self, value):
-        self._set_attr('max-age', str(value))
+        if value is not None:
+            self._set_attr('max-age', str(value))
 
     def get_data(self):
         return base64.b64decode(bytes(self.xml.text))