Cannot overwrite message callback

[sticmac]

When using ColyseusRoom.OnMessage twice on the same message ID, I get the following exception:

ArgumentException: An item with the same key has already been added. Key: myMessage

An easy fix would be to modify the OnMessage method to take in consideration if a previous message with the given id already exists. However, I wanted to know if this is a bug or a real design choice before submitting a PR.

[endel]

Thanks for reporting, @sticmac,

We've added support for multiple callbacks on the other clients for upcoming version 0.15 but it turns out the Unity SDK is still missing this feature (on #177)

Ideally, the Unity SDK should be able to attach and detach multiple callbacks, for example:

var detachCallback1 = room.OnMessage("xx", () => { /* first */ });
var detachCallback2 = room.OnMessage("xx", () => { /* second */ });

// detaching the "OnMessage" callback:
detachCallback1();
detachCallback2();

[sticmac]

I made a pull request (#199) containing a solution like the example you provide :)