Server. Fix server close error

[sgrebnov]

After making HttpSocketServer .ctr not to block the calling thread the following error was found when HttpSocketServer.Dispose is called.

Cannot access a disposed object.Object name: 'System.Net.Sockets.Socket'

Repro steps

1. Update to the latest DevBranch codebase
2. Run Tests\Http2.Owin.Server.WebApi.Tests\Http2.Owin.Server.WebApi.Tests
3. Press enter
   Expected: server is closed with no error
   Actual: the error above is raised

[sgrebnov]

Verified, closing