internal/health: use http.Get instead of s.Client.Get

httptest.Server.Client was added in Go 1.9, and we want to support Go
1.8+ for App Engine.

Change-Id: I1eb539a69c2dadc85505b181eeeb6e20dd333df7
Reviewed-on: https://go-review.googlesource.com/78539
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/internal/health/health_test.go b/internal/health/health_test.go
index 8c7893d..5a68d7e 100644
--- a/internal/health/health_test.go
+++ b/internal/health/health_test.go
@@ -67,7 +67,7 @@
 }
 
 func check(s *httptest.Server) (code int, err error) {
-	resp, err := s.Client().Get(s.URL)
+	resp, err := http.Get(s.URL)
 	if err != nil {
 		return 0, err
 	}