InvitationSerializer: fix user validator.

django_etesync/serializers.py

@@ -282,13 +282,16 @@ class CollectionInvitationSerializer(serializers.ModelSerializer):
     def get_from_pubkey(self, obj):
         return b64encode(obj.fromMember.user.userinfo.pubkey)
 
-    def create(self, validated_data):
-        collection = self.context['collection']
+    def validate_user(self, value):
         request = self.context['request']
 
-        if request.user == validated_data.get('user'):
+        if request.user == value:
             raise serializers.ValidationError('Inviting yourself is not allowed')
 
+    def create(self, validated_data):
+        collection = self.context['collection']
+        request = self.context['request']
+
         member = collection.members.get(user=request.user)
 
         with transaction.atomic():