[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/__snapshots__/qweb_t_att.test.ts.snap

@@ -0,0 +1,29 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`qweb t-att t-att-class with multiple classes 1`] = `
+"function anonymous(context, extra
+) {
+    // Template name: \\"test\\"
+    let utils = this.constructor.utils;
+    let scope = Object.create(context);
+    let h = this.h;
+    let _1 = utils.toClassObj({'a b c':scope['value']});
+    let c2 = [], p2 = {key:2,class:_1};
+    let vn2 = h('div', p2, c2);
+    return vn2;
+}"
+`;
+
+exports[`qweb t-att t-att-class with multiple classes 2`] = `
+"function anonymous(context, extra
+) {
+    // Template name: \\"test\\"
+    let utils = this.constructor.utils;
+    let scope = Object.create(context);
+    let h = this.h;
+    let _3 = utils.toClassObj({['a b c']:scope['value']});
+    let c4 = [], p4 = {key:4,class:_3};
+    let vn4 = h('div', p4, c4);
+    return vn4;
+}"
+`;