Slug is not generated as unique in 0.6

Steps to reproduce

• Make sure you have an article called "Test" with slug "test"
• Add a new article called "Test"
• 500 error:
  [2017-02-01 12:17:05] request.CRITICAL: Uncaught PHP Exception MongoDuplicateKeyException: "localhost:27017: E11000 duplicate key error index: integrated.content.$slug_1 dup key: { : "test-2" }" at /home/integrated/releases/20170131163348/vendor/doctrine/mongodb/lib/Doctrine/MongoDB/Collection.php line 1022 {"exception":"[object] (MongoDuplicateKeyException(code: 11000): localhost:27017: E11000 duplicate key error index: integrated.content.$slug_1 dup key: { : \"test-2\" } at /home/integrated/releases/20170131163348/vendor/doctrine/mongodb/lib/Doctrine/MongoDB/Collection.php:1022)"} []

Cause

0.5 does not seem to have this error. It may be caused by the mapping from annotations to XML conversion in the ContentBundle. The XML driver seems not to be implemented yet: slug-bundle\Mapping\Driver\XmlDriver.php