Fix collection thumbnails

[sammeltassen]

When iiif-hss processes IIIF Collection Manifests, it seems to alter the embedded thumbnails. Taking the portrait of J.E.N. baron Schimmelpenninck as an example:

• Input collection manifest
  • Thumbnail (Correct)
  • Collection page
• Input manifest (No thumbnail property)
• Output collection
  • Thumbnail (Incorrect)
• Output manifest (No manifest thumbnail!)
  • Object page

In the collection overview the last image of the manifest is used as thumbnail (in this case the back of the painting):

[sammeltassen]

For some reason this seems to have solved itself after a new build...?