[FIX] qweb: fix overlapping multi-class in t-att-class

Previously, if two attributes in t-att-class shared some classes, their
presence would be determined by the last attribute declared, instead of
being present if any attribute containing it evaluates to true. This
commit fixes that.

tests/qweb/qweb_t_att.test.ts

@@ -24,4 +24,13 @@ describe("qweb t-att", () => {
       '<div class="a b c"></div>'
     );
   });
+
+  test("t-att-class with multiple classes, some of which are duplicate", () => {
+    expect(render(`<div t-att-class="{'a b c': value, 'a b d': !value}" />`, { value: true })).toBe(
+      '<div class="a b c"></div>'
+    );
+    expect(render(`<div t-att-class="{'a b c': value, 'a b d': !value}" />`, { value: false })).toBe(
+      '<div class="a b d"></div>'
+    );
+  });
 });