cmd/kivik: open_revs query fails with error

[flimzy]

$ kivik get https://admin:abc123@localhost:6007/foo/bar?open_revs=all
Error: kivik: invalid content type in response: multipart/mixed

The same with curl:

$ curl -v -X GET https://admin:abc123@localhost:6007/foo/bar?open_revs=all
Note: Unnecessary use of -X or --request, GET is already inferred.
*   Trying 127.0.0.1:6007...
* Connected to localhost (127.0.0.1) port 6007 (#0)
* Server auth using Basic with user 'admin'
> GET /foo/bar?open_revs=all HTTP/1.1
> Host: localhost:6007
> Authorization: Basic YWRtaW46YWJjMTIz
> User-Agent: curl/7.88.1
> Accept: */*
> 
< HTTP/1.1 200 OK
< Content-Type: multipart/mixed; boundary="123ee7d3a48a9bee3742680fe4fcf7f9"
< Date: Mon, 15 Apr 2024 07:58:24 GMT
< Server: CouchDB/3.3.3 (Erlang OTP/24)
< Transfer-Encoding: chunked
< X-Couch-Request-ID: 7c24c2b5a6
< X-CouchDB-Body-Time: 0
< 
--123ee7d3a48a9bee3742680fe4fcf7f9
Content-Type: application/json

{"_id":"bar","_rev":"5-b437ea2513e4a09882422b9972463f37"}
* Connection #0 to host localhost left intact
--123ee7d3a48a9bee3742680fe4fcf7f9--