[FIX] qweb: prevent issue with builtin object properties

The QWeb expression parser use an object as a mapping between some
strings and the desired output in the compiled template.  However, as we
should all know, objects are not Maps, they have some additional
properties, such as "constructor" or "hasOwnProperty".

The simple solution is to make sure the mapping object does not have
anything in its prototype chain to pollute its purpose.

closes #835

tests/qweb/qweb_expressions.test.ts

@@ -196,4 +196,8 @@ describe("expression evaluation", () => {
     expect(compileExpr("f(...state.list)", {})).toBe("scope['f'](...scope['state'].list)");
     expect(compileExpr("f([...list])", {})).toBe("scope['f']([...scope['list']])");
   });
+
+  test("works with builtin properties", () => {
+    expect(compileExpr("state.constructor.name", {})).toBe("scope['state'].constructor.name");
+  });
 });