Fix CollecionVersion::getAttribute Cache Issue

Fix `CollectionVersion::getAttribute()` Cache Issue

`CollectionVersion::getAttribute()` caches attributes in an array based
on the attribute handle `akHandle`

When a Collection Attribute is requested with two different display
modes `$displayMode` in the same request the first one requested will
always be returned.

- Add `$akHash` value comprised of both attribute handle and display
  mode for hash key
Merge pull request #1520 from mkly/fix_collection_version_get_attribute_cache

Fix CollecionVersion::getAttribute Cache Issue