Enable SSL in FoundationTransport for secure hosts

[nickdowell]

This fixes connection to secure hosts such as wss://echo.websocket.org

The kCFStreamPropertySocketSecurityLevel property needs to be set for the CFStream(s) to negotiate a secure connection with the server.

• Fixes Cannot connect to secure wss:// websocket on iOS 11 & earlier #748
• Fixes WebSocket instantly disconnect on iOS 10 using Starscream 4.0.0 #728

[michalcichon]

Hey @nickdowell, can you paste here what kind of error do you get because of this bug?

[nickdowell]

Hey @michalcichon the bug causes connections to secure hosts to fail with the following:

WebSocketEvent.cancelled

Sample code for reproducing the issue was included in #748

[daltoniam]

Thanks!