Make it possible to escape special characters on serialization

[Actine]

It's a good practice to convert the following special characters: < > " & ' into their respective hex representation: \u003C, \u003E, \u0022, \u0026 and \u0027: it mitigates some XSS while remaining valid JSON.
Symfony2's JsonResponse does this escaping, don't see why JMSSerializer shouldn't.
Sorry if dup bug.

[Actine]

Sorry, false alarm. Read some docs, found the way.
It can be configured in the following way:

# config.yml or wherever
jms_serializer:
    visitors:
        json:
            options: 15

where 15 is a bitmask of JSON_HEX_TAG + JSON_HEX_AMP + JSON_HEX_APOS + JSON_HEX_QUOT