http: fix incorrect prints found by govet

R=golang-dev, bradfitzgo
CC=golang-dev
https://golang.org/cl/4370053

src/pkg/http/transport_test.go

@@ -288,10 +288,10 @@ func TestTransportHeadResponses(t *testing.T) {
 			t.Errorf("error on loop %d: %v", i, err)
 		}
 		if e, g := "123", res.Header.Get("Content-Length"); e != g {
-			t.Errorf("loop %d: expected Content-Length header of %q, got %q", e, g)
+			t.Errorf("loop %d: expected Content-Length header of %q, got %q", i, e, g)
 		}
 		if e, g := int64(0), res.ContentLength; e != g {
-			t.Errorf("loop %d: expected res.ContentLength of %v, got %v", e, g)
+			t.Errorf("loop %d: expected res.ContentLength of %v, got %v", i, e, g)
 		}
 	}
 }