[FIX] qweb: handle variable expressions in t-if

closes #390
closes #362

src/qweb/qweb.ts

@@ -645,7 +645,7 @@ export class QWeb extends EventBus {
       if (name.startsWith("t-att-")) {
         let attName = name.slice(6);
         const v = ctx.getValue(value);
-        let formattedValue = v.id || ctx.formatExpression(v);
+        let formattedValue = typeof v === 'string' ? ctx.formatExpression(v) : v.id;
 
         if (attName === "class") {
           ctx.rootContext.shouldDefineUtils = true;