Update Cart.php

[OscarKolsrud]

Fixed a bug where 0 is passed as the taxrate to the Cart:add facade resulting in it not evaluating right since '0' is not true in a if statement. Changed therefore the comparison to isset() so it rather checks if there is passed a taxrate of some kind. Also added in the if statement a simple check to see if the taxrate is numeric, and if not default to the one in the config

[hardevine]

thanks

[OscarKolsrud]

Thanks for merging @hardevine . Maybe you can also publish the update on packagist so it becomes available easily in composer?