[FIX] web: MockServer: don't alter domain in place

In a test, we might want to check the domains given to RPCs with
assert.step/verifySteps. Before this rev., if there were an
'active' field on the model, ['active', '=', true] was
automatically added to the domain by the MockServer in place, thus
altering the expected domain in the test.

addons/web/static/tests/helpers/mock_server.js

@@ -335,7 +335,7 @@ var MockServer = Class.extend({
                 activeInDomain = activeInDomain || subdomain[0] === 'active';
             });
             if (!activeInDomain) {
-                domain.unshift(['active', '=', true]);
+                domain = [['active', '=', true]].concat(domain);
             }
         }