Invalid json check

Still need test for `hasJsonKeyValue`.

To run tests, you need to:
1. Update a project to use this version of the repo
2. Run `php composer.phar update UseAllFive/admin-theme-plugin`
3. Update `config/ProjectConfiguration.class.php` to use `sfTaskExtraPlugin`
4. Run the test:
```bash
SYMFONY=lib/vendor/symfony/symfony1/lib php plugins/ua5AdminThemePlugin/test/unit/test/Ua5TesterResponseTest.php
```