services/dropbox: WriteAppend test fails with not_found/... error

[Joey-1445601153]

Post "https://content.dropboxapi.com/2/files/upload_session/append_v2": EOF

ref: https://teamcity.beyondstorage.io/buildConfiguration/Services_Dropbox_IntegrationTests/878?buildTab=log&focusLine=237&expandAll=true&logFilter=debug

[Xuanwo]

Not reproduced, let's reopen it if the problem happened again.