Unable to send message

Sending a message raises an exception:
```
raised CONSTRAINT_ERROR : namespace is not mapped to any prefix
```

This could be fixed by commenting this lines in (xmpp-messages.adb Serialize): 
```
      --  setting xml:lang attr
      Attrs.Set_Value
       (Namespace_URI => XML_URI,
        Local_Name    => Lang_Attribute,
        Value         => Self.Language);
```