Restore undetermined state in Cache configuration

[jderusse]

This PR fix a BC break introduced by #707.

Given the following code:

    /**
     * @Cache(smaxage="+1min")
     */
    public function fooAction(){}

The headers generated by HttpCacheListener changed as following:

- public, s-maxage=60
+ private, s-maxage=60

Why? Because previously the Cache's properties were null until the user provide a value. In such state, isPublic returns false (although isPrivate). And sometimes, null is expected (like the vary array =>

SensioFrameworkExtraBundle/src/EventListener/HttpCacheListener.php

Line 135 in b359690

if (!$response->headers->has('Vary') && null !== $configuration->getVary()) {

)

The #707 PR introduce 2 changes:

• a value for each property is now forced by the constructor's default value.
• the constructor calls the setter for each property (even if the user did not provide a value for it), some setter (ie. setPublic) does not expect a null value and convert null into something.

It looks like, only the Cache configuration is affected. Other Configuration has already a default value defined for each property.

This PR restore the default null value for all properties and prevent the constructor to call the setter when the value is null.

[fabpot]

Thank you @jderusse.