Fix EZP-21278: [Rest API] Get current version with anonymous user spec don't correspond to behaviour

This PR resolves issue https://jira.ez.no/browse/EZP-21278

Couple of problems solved here:

* `ContentService::loadVersionInfoById( $contentId, $versionNo )` checked `content/versionread` in all cases (even if current version number was given)
* Missing unit tests for `ContentService::loadVersionInfoById()`
* Unnecessary loading of both ContentInfo and VersionInfo in `REST/Server/Controller/Content::redirectCurrentVersion()`