Edition.add_bookcover() returning 500 response for valid urls

Troubleshooting [CoverBot](https://github.com/internetarchive/openlibrary-bots/tree/master/coverbot) revealed the following problem with `add_bookcover`

```
Python 3.9.4 (default, Apr 9 2021, 01:15:05)
>>> from olclient.openlibrary import OpenLibrary
>>> ol = OpenLibrary()
>>> obj = ol.Edition.get('OL25511397M')
>>> obj.cover
>>> obj.ocaid
'elentremetidocom00gily' # note the correct ocaid is elentremetidocom00gily_0
>>> obj.add_bookcover('https://archive.org/download/elentremetidocom00gily_0/page/cover')

```
https://archive.org/download/elentremetidocom00gily_0/page/cover is a valid url and the upload url used in `add_bookcover` resolves to https://openlibrary.org/books/OL25511397M/-/add-cover for this edition, which is also valid.

When I try to manually add a cover from the above upload url, I get an error. The url I land on is: https://openlibrary.org/books/OL25511397M/El_entremetido_comedia_en_tres_actos_n_!_prosa/add-cover

I suspect that root cause of this problem isn't with the client, but with OpenLibrary's backend, but I'm opening the issue here