http_test: simplify strings.Index with strings.Contains

Both have the same O(n), but strings.Contains more readable on
if-condition.

http_test.go

@@ -395,7 +395,7 @@ func TestRequestUpdateURI(t *testing.T) {
 	if !strings.HasPrefix(s, "GET /123/432.html?aaa=bcse") {
 		t.Fatalf("cannot find %q in %q", "GET /123/432.html?aaa=bcse", s)
 	}
-	if strings.Index(s, "\r\nHost: foobar.com\r\n") < 0 {
+	if !strings.Contains(s, "\r\nHost: foobar.com\r\n") {
 		t.Fatalf("cannot find %q in %q", "\r\nHost: foobar.com\r\n", s)
 	}
 }