fix Future exception never retrieved in create_connection

[jensbjorgensen]

I also have observed that when I use asyncio.wait_for(...) around loop.create_connection in the timeout case I end up getting this logged to stdout(err?):

Future exception was never retrieved
future: <Future finished exception=gaierror(-2, 'Name or service not known')>
socket.gaierror: [Errno -2] Name or service not known

The code in this commit fixes this.

[fantix]

This makes sense! Though a unit test is preferred.

[fantix]

Why dropping the original context here, instead of just raise?