[FIX] qweb: do not override t-att-class with class

There was some code in qweb to make sure that we support setting class
and t-att-class on the same html element:

<div class="some class" t-att-class="{b: true}">...</div>

But the code did not work in the other direction:

<div t-att-class="{b: true}" class="some class">...</div>

With this commit, we just add the missing if statement

closes #664

src/qweb/qweb.ts

@@ -704,8 +704,12 @@ export class QWeb extends EventBus {
               .split(/\s+/)
               .map(a => `'${escapeQuotes(a)}':true`)
               .join(",");
-            classObj = `_${ctx.generateID()}`;
-            ctx.addLine(`let ${classObj} = {${classDef}};`);
+            if (classObj) {
+              ctx.addLine(`Object.assign(${classObj}, {${classDef}})`);
+            } else {
+              classObj = `_${ctx.generateID()}`;
+              ctx.addLine(`let ${classObj} = {${classDef}};`);
+            }
           }
         } else {
           ctx.addLine(`let _${attID} = '${escapeQuotes(value)}';`);

tests/qweb/__snapshots__/qweb.test.ts.snap

@@ -1,5 +1,20 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`attributes class and t-att-class should combine together 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.toObj(scope['value']);
+    Object.assign(_1, {'hello':true})
+    let c3 = [], p3 = {key:3,class:_1};
+    let vn3 = h('div', p3, c3);
+    return vn3;
+}"
+`;
+
 exports[`attributes class and t-attf-class with ternary operation 1`] = `
 "function anonymous(context, extra
 ) {

tests/qweb/qweb.test.ts

@@ -715,6 +715,12 @@ describe("attributes", () => {
     expect(result).toBe(`<div class="hello world"></div>`);
   });
 
+  test("class and t-att-class should combine together", () => {
+    qweb.addTemplate("test", `<div t-att-class="value" class="hello" />`);
+    const result = renderToString(qweb, "test", { value: "world" });
+    expect(result).toBe(`<div class="world hello"></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 });