HubProxy.Subscribe is not thread safe

[MansVestin]

Expected behavior

HubProxt.Subscribe(eventName) should be thread safe, according to documentation here:
https://www.asp.net/signalr/overview/guide-to-the-api/hubs-api-guide-net-client
And here:
http://stackoverflow.com/questions/16089876/signalr-hubproxy-invoke-thread-safety

(However, it's stated to be non-thread safe here:
https://msdn.microsoft.com/en-us/library/microsoft.aspnet.signalr.client.hubs.hubproxy(v=vs.111).aspx)

Actual behavior

HubProxy.Subscribe() accesses the _subscriptions list in a non-thread safe manner. This causes data to be sent to the wrong onData callback.

Steps to reproduce

If you call IHubProxy.On(eventName, onData) (which in turn calls HubProxy.Subscribe) lots of times from different threads at the same time, data from one event may be delivered to another onData callback.
Adding a lock around the On() call prevents the issue.

[moozzyk]

This seems like the old client. Please open in http://github.com/SignalR/SignalR.