oa:Annotation['on'] not present and required

[minusdavid]

According to https://github.com/IIIF/presentation-validator and http://iiif.io/api/presentation/2.0/#image-resources, "Although it seems redundant, the URI of the canvas must be repeated in the on field of the Annotation."

This is missing from O'Sullivan at the moment...

At a glance, this should be easy enough to add in. It's 4:32pm here... I might try hacking on the O'Sullivan code and put in a pull request if I can get it working.

[minusdavid]

Oh that was surprisingly easy to implement. Pull request to come soon...