allows managers to access embargoed objects
Fix logic to allow embargoed objects thumbnail datastreams through.

Now returns true if the object being requested is a thumbanail datastream from another objects page, like on a collection display page.

Also allows embargoed object datastreams through even if a redirect URL has not been
set.
Merge pull request #2 from lyrasis/fix-embargoed-tn-datastream

Fix embargoed object  thumbnail datastream display