Fix typo in IMAGE_META_HEADERS

x-image-meta-deleted-at replaced with x-image-meta-deleted_at
This prevents a problem when running the add_image method.

The bug was entered as part of the following commit:
c3e58bd943ccb4e1457914c3c2b5366bd0afee5a

Closes-Bug:#1220067
Change-Id: I6d1624a00e89721f5c9af969e191951fdcb2ee43
Merge "Fix typo in IMAGE_META_HEADERS"