net/http: fix goroutine leak in benchmark

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/3412
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/serve_test.go

@@ -2977,6 +2977,7 @@ func BenchmarkClient(b *testing.B) {
 	case <-time.After(5 * time.Second):
 		b.Fatalf("subprocess did not stop")
 	}
+	DefaultTransport.(*Transport).CloseIdleConnections()
 }
 
 func BenchmarkServerFakeConnNoKeepAlive(b *testing.B) {