[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.

src/qweb/qweb.ts

@@ -127,12 +127,12 @@ const UTILS: Utils = {
       return result;
     }
     // this is already an object, but we may need to split keys:
-    // {'a': true, 'b c': true} should become {a: true, b: true, c: true}
+    // {'a b': true, 'a c': false} should become {a: true, b: true, c: false}
     for (let key in expr) {
       const value = expr[key];
       const words = key.split(/\s+/);
       for (let word of words) {
-        result[word] = value;
+        result[word] = result[word] || value;
       }
     }
     return result;