Race builders report goroutine leaks after addition of this benchmark:
http://build.golang.org/log/18e47f4cbc18ee8db125e1f1157573dd1e333c41
Close idle connection in default transport.

Change-Id: I86ff7b2e0972ed47c5ebcb9fce19e7f39d3ff530
Reviewed-on: https://go-review.googlesource.com/3412Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>