fix: allow t-attf- attributes to combine with regular attrs

fixes #10

tests/__snapshots__/qweb.test.ts.snap

@@ -207,6 +207,17 @@ exports[`attributes t-att-class with object 1`] = `
 }"
 `;
 
+exports[`attributes t-attf-class should combine with class 1`] = `
+"function anonymous(context,extra
+) {
+    let h = this.utils.h;
+    let _1 = 'hello ' + \`world\`;
+    let c2 = [], p2 = {key:2,attrs:{class: _1}};
+    let vn2 = h('div', p2, c2);
+    return vn2;
+}"
+`;
+
 exports[`attributes tuple literal 1`] = `
 "function anonymous(context,extra
 ) {

tests/qweb.test.ts

@@ -401,6 +401,12 @@ describe("attributes", () => {
     expect(result).toBe(`<div foo="bar"></div>`);
   });
 
+  test("t-attf-class should combine with class", () => {
+    qweb.addTemplate("test", `<div class="hello" t-attf-class="world"/>`);
+    const result = renderToString(qweb, "test");
+    expect(result).toBe(`<div class="hello world"></div>`);
+  });
+
   test("format value", () => {
     qweb.addTemplate("test", `<div t-attf-foo="b{{value}}r"/>`);
     const result = renderToString(qweb, "test", { value: "a" });