[FIX] qweb: enclose interpolated expr in parentheses

Since rev 53e53e9b,
 <div class="o_app" t-attf-class="{{ true ? 'o_extra' : '' }}"/>
produced
 <div class="o_extra"/>
whereas it should have produced
 <div class="o_app o_extra"/>

This is because there were missing parentheses around the string
returned by `interpolate`, which produced instructions like

 var _1 = 'o_app ' + true ? 'o_extra' : '';

Fixes #132

tests/qweb.test.ts

@@ -522,6 +522,12 @@ describe("attributes", () => {
     expect(result).toBe(`<div class="hello world"></div>`);
   });
 
+  test("class and t-attf-class with ternary operation", () => {
+    qweb.addTemplate("test", `<div class="hello" t-attf-class="#{value ? 'world' : ''}"/>`);
+    const result = renderToString(qweb, "test", { value: true });
+    expect(result).toBe(`<div class="hello world"></div>`);
+  });
+
   test("t-att-class with object", () => {
     qweb.addTemplate(
       "test",