WS-601: addresses bug in additional_metadata section of manifest that has a...

WS-601: addresses bug in additional_metadata section of manifest that has a class `repr` rather than a JSON-serializable value

* tests dump all JSON strings; if dumps succeed, the JSON is good
* removed contrived test case
* added test case based on what an image ingestion staging dir is likely to contain

TODO next MR:
additional tests for new image ingestion test case