[IMP] qweb: allow multiclasses in t-att-class object form

the low level method htmlelement.classList.add does not accept multiple
classes in one string, which is why, in owl, the expression

`<div t-att-class="{'a b c': value}" />`

did not work as one might expect. It is however very convenient in real
life templates, so this commit improve owl by adding support for this
feature.

closes #813

tests/qweb/qweb_t_att.test.ts

@@ -0,0 +1,27 @@
+import { QWeb } from "../../src/qweb/index";
+import { renderToString } from "../helpers";
+
+//------------------------------------------------------------------------------
+// Setup and helpers
+//------------------------------------------------------------------------------
+
+function render(template, context = {}) {
+  const qweb = new QWeb();
+  qweb.addTemplate("test", template);
+  return renderToString(qweb, "test", context);
+}
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+describe("qweb t-att", () => {
+  test("t-att-class with multiple classes", () => {
+    expect(render(`<div t-att-class="{'a b c': value}" />`, { value: true })).toBe(
+      '<div class="a b c"></div>'
+    );
+    expect(render(`<div t-att-class="{['a b c']: value}" />`, { value: true })).toBe(
+      '<div class="a b c"></div>'
+    );
+  });
+});