Tls 1.2 support

[JanOudkerk]

Our software calls the PusherClient dll from unmanaged code (C++). As a result it uses TLS 1.0 as its security protocol.
For some unknown reason this is the default when a .net component is called from unmanaged code.

We resolved this by modifying WebSocket's constructor in Connection.cs to set it to SslProtocols.Tls12.

This is a quick fix that works for us.
Probably better to put this setting in the PusherOptions object?

[JanOudkerk]

We implemented these changes in fork https://github.com/JanOudkerk/pusher-websocket-dotnet.git
(I wasn't succesful in sending a pull request to this repo)

[stale]

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[JanOudkerk]

We created a pull request for this