Only send Strict Transport Security Header using https

If AppInventorFeatures.enableHttpRedirect() is false, don't send the
Strict-Transport-Security header. Chrome appears to ignore it when it
is sent over http instead of https, but I wouldn't want to depend on
that behavior!

Change-Id: I585fc6164a57d7502b737c4596cbd42b84489dd2

appinventor/appengine/war/index.jsp

@@ -13,7 +13,9 @@
         }
      return;
    }
+   if (AppInventorFeatures.enableHttpRedirect()) {
        response.setHeader("Strict-Transport-Security", "max-age=3600");
+   }
 %>
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