GetCustomVariable - Invalid result for Visit and Event scoped custom variables

[julienmoumne]

Discovered by @ptr1120 in #36

Querying for an Event scoped custom variable results in an exception.

Querying for a Visit scoped custom variable attempts to return an equivalently named Event scoped custom variable.

The only valid case is querying for a Page scoped custom variable.

The bug was introduced here caba042#diff-3ef10527762c9550c23f3f369808ffa6R290

The test should not be negated as in https://github.com/piwik/piwik-php-tracker/blob/master/PiwikTracker.php#L294