Fix FilesService.Upload

[michaelfeinbier]

Hey there,
in this GitRepo you do not have the "Issues" activated to report a potential bug so I tried to investigate myself.

For me, the FilesService.Upload does not work correctly.

putio error. code:404 type:"" message:"" request:POST https://upload.put.io

After investigating, I found out that according to the API specifications of putio the Endpoint to upload should be https://upload.put.io/v2/files/upload instead of just https://upload.put.io. This is what the code is doing. Until now, we're using a weird placeholder $upload$ to override the hostname. I improved the logic, made clearer what is happening and added tests to the client

I did not touch the logic of func (u *UploadService) CreateUpload( this uses a weird placeholder as well. I do not know where this endpoint is reflected in V2 at all, so I kept the logic as is here

[cenkalti]

Hi @michaelfeinbier . Thanks for reporting this and your contribution. I've fixed this issue by just changing the upload URL with this commit: 873b1f4

Please let me know if it does not resolve your problem.