Remove SerializableClosure security provider

[shrimpwagon]

If a secret key was set on the SerializableClosure then any future closure will be signed. This occurs in the Laravel framework. The child processes are not ran in Laravel thus not using the secret key that was set thus failing. This edit will temporarily remove the security provider, create an unsigned closure and set the provider back to original.

[msarca]

This PR won't fix the issue. The signing occurs at serialization time, not when the SerializableClosure class is instantiated. Moreover, if the value returned by the getSecurityProvider method is null, an exception will be thrown when passing null as an argument to the addSecurityProvider method. The correct solution was already posted.

[brendt]

So I guess I'll close this one, as this seems to be the solution: #52 (comment)