I haven't been able to reproduce this one, but change a few suspect
things in this test. Notably, using the global "Get" function and thus
using the DefaultTransport was buggy in a parallel test. Then add some error
checks and close a TCP connection.

Hopefully the failure wasn't timing-related.

Fixes #18036 (I hope)

Change-Id: I4904e42e40b26d488cf82111424a1d4d46f42dae
Reviewed-on: https://go-review.googlesource.com/34490
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>