Save_many fails for specific set of work IDs

When using colab to migrate these work editions OL15037366W,OL15037371W,OL16056627W into OL73087W
an error is thrown.

### Evidence / Screenshot (if possible)

### Relevant url?
https://colab.research.google.com/drive/1OdyL5nPf4PEaWavuxCDYs9q2C4JKrP90#scrollTo=oODoBD3kHk6v&line=1&uniqifier=1
### Steps to Reproduce

1. Go to ... https://colab.research.google.com/drive/1OdyL5nPf4PEaWavuxCDYs9q2C4JKrP90#scrollTo=oODoBD3kHk6v&line=1&uniqifier=1
2. Do
`save_set = []
for work in works:
editions = work.editions
for ed in editions:
ed.work_olid = into_olid
save_set.append(ed)
response = ol.save_many(save_set, 'moving edition(s) to primary work')
print('save_set:',save_set)
response.raise_for_status()
response`

* Actual:
```
HTTPError Traceback (most recent call last)
in ()
7 response = ol.save_many(save_set, 'moving edition(s) to primary work')
8 print('save_set:',save_set)
----> 9 response.raise_for_status()
10 response
```

/usr/local/lib/python3.7/dist-packages/requests/models.py in raise_for_status(self)
941
942 if http_error_msg:
--> 943 raise HTTPError(http_error_msg, response=self)
944
945 def close(self):

HTTPError: 400 Client Error: Bad Request for url: https://openlibrary.org/api/save_many
* Expected:

### Details

- **Logged in (Y/N)?**
- **Browser type/version?**
- **Operating system?**
- **Environment (prod/dev/local)?** prod

### Proposal & Constraints

The script works for other sets of Works. Just this set throws the error.
### Related files

### Stakeholders

@cdrini