Update FirebaseToken.php

when no option is given, the line below may raise compiler error (depending on the errorlevel set) because $claims["admin"] is undefined:

$this->_validateData($funcName, $data, ($claims["admin"] == true));

I changed it so that it defaults to full access as I guess it is the wanted default behavior.
Thanks for contributing this! Great stuff.

I think `($claims["admin"] == true)` would actually default to false, unless that value was explicitly `true`. This should probably default to `false` here for consistency.