Fix cache error checking

[gabrielsoldani]

This adds a condition to check if the Snapchat::$cache object is set before using it. If it is not set, it issues the request normally and does not cache the response, if successful.

The second commit adds a check before setting the cache as well. The request should not normally complete successfully when $cache is not set, but in the future you might want to add the option to not to set the cache object in the constructor/login so requests are never cached, and this would be necessary.