TestAttachmentsRevisions and TestAttachmentsSession flaky on mac

[kjk]

Tested with 4.1.3 (was same with 4.0.6)

Logs from successful go run: https://gist.github.com/kjk/1e7bc937bd56b060cb8ab952e1732260
Logs from failed go run: https://gist.github.com/kjk/34460dbd4d18f099290ad22f4259d42a
Logs from failed java run: https://gist.github.com/kjk/d9f7808a1aa80103f1994b475fddfffc

Java version is also flaky.

Doesn't seem to happen on Windows or Linux on AppVeyor/Travis.

Seems like a server bug. Sometimes the server fails to handle PUT /databases/test_db_1/attachments?id=users%2F1&name=profile.png&contentType=image%2Fpng HTTP/1.1 command.

There's nothing enlightening in the server logs.