XmlRpcStruct | Possible duplicate key in private ArrayList

[titobrasolin]

In my opinion there is a problem in this override: https://github.com/snielsson/XmlRpcLight/blob/master/XmlRpcLight/DataTypes/XmlRpcStruct.cs#L21

Setting an item always adds a new _key e.g.

    var x = new XmlRpcStruct ();
    x ["title"] = "Hi!";
    x ["title"] = "Hi again!";
    x ["title"] = "Hi (third time)!";

You will end up with a single "title" in the base class but three items in the private _keys.
This can possibly lead to serialization problems.

Possible solution: check if the key already exists e.g. after line 31

    var indexOfKey = _keys.IndexOf (key);
    if (indexOfKey < 0) {
        _keys.Add (key);
        _values.Add (value);
    } else {
        _values [indexOfKey] = value;
    }