Make WebSocketConnection.Connect virtual

[Shane32]

I'm trying to write an alternate implementation of IWebSocketConnectionFactory<TSchema> and cannot do so without this change.

For the next major version, IWebSocketConnectionFactory.CreateConnection should return an interface instead of a class.

[codecov-commenter]

Codecov Report

Merging #674 (8df7dc2) into master (fe16988) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #674   +/-   ##
=======================================
  Coverage   55.02%   55.02%           
=======================================
  Files          64       64           
  Lines        1592     1592           
  Branches      158      158           
=======================================
  Hits          876      876           
  Misses        662      662           
  Partials       54       54           

Impacted Files	Coverage Δ
...ts.Subscriptions.WebSockets/WebSocketConnection.cs	0.00% <ø> (ø)

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