Fixes flash messaging/SessionCookie decoding failure when cookies.encrypt = true

Without a way to check if a cookie is already encoded, `\Slim\Http\Util::serializeCookies()` will encode an already encoded cookie. This is an issue when `\Slim\Middleware\SessionCookie` sets an encoded cookie value on the response object during `\Slim\Middleware\SessionCookie::saveSession()` (as `\Slim\Http\Util::serializeCookies()` will encode every cookie it gets if `cookies.encrypt` = `true`).  Since the cookie is only decoded once during `\Slim\Middleware\SessionCookie::loadSession()`, the cookie's actual decoded value is never available to the application, and `unserialize()` in `\Slim\Middleware\SessionCookie::loadSession()` will fail.

This PR adds the ability to tell `\Slim\Http\Util::serializeCookies()` to skip encoding a cookie during the serialization process.