Revert serializing and string casting in Storage::set().

This should fix the problem stated [here](https://github.com/hybridauth/hybridauth/issues/200#issuecomment-50018813). The imploding of value breaking the OpenID provider and most likely others too as there's not corresponding exploding of values when they are read back from session. I am pretty sure all the other error reports for "Illegal string offset" are also as a result of this.

We'll need to come up with a better fix for the original "Serialization of 'Closure' is not allowed" error that these changes intended to solved.