Push notifications don't work properly for m.rule.member_event

### This is a low priority issue

Was messing around with https://matrix.org/docs/spec/client_server/r0.6.0#m-rule-member-event after looking at https://github.com/vector-im/riot-web/issues/2104

Riot iOS doesn't show the correct message when the `m.rule.member_event` push rule is set like:
```
{
"conditions": [
{
"kind": "event_match",
"key": "type",
"pattern": "m.room.member"
}
],
"actions": [
"notify",
{
"set_tweak": "sound",
"value": "default"
},
{
"set_tweak": "highlight"
}
],
"rule_id": ".m.rule.member_event",
"default": true,
"enabled": true
}
```

The push notification shows User has invited you to Room no matter what the member event is.

For example this user changed their display name but Riot iOS says that I have been invited to the room