[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

src/component/directive.ts

@@ -291,7 +291,7 @@ QWeb.addDirective({
       if (tattClass) {
         let tattExpr = ctx.formatExpression(tattClass);
         if (tattExpr[0] !== "{" || tattExpr[tattExpr.length - 1] !== "}") {
-          tattExpr = `utils.toObj(${tattExpr})`;
+          tattExpr = `utils.toClassObj(${tattExpr})`;
         }
         if (classAttr) {
           ctx.addLine(`Object.assign(${classObj}, ${tattExpr})`);