net/http: fix test assertion

Logf doesn't make the test fail, so the test was always OK.

Change-Id: I7c10ee74ff7e5d28cbd3a35e185093cb9f349470
Reviewed-on: https://go-review.googlesource.com/120496
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/transfer_test.go

@@ -86,7 +86,7 @@ func TestDetectInMemoryReaders(t *testing.T) {
 	for i, tt := range tests {
 		got := isKnownInMemoryReader(tt.r)
 		if got != tt.want {
-			t.Logf("%d: got = %v; want %v", i, got, tt.want)
+			t.Errorf("%d: got = %v; want %v", i, got, tt.want)
 		}
 	}
 }