Windows-style backslashes appear in s3 paths when using recursive upload

s4cmd should perform path separator conversion to forward slashes before uploading. For example, when calling `s4cmd -r put dir s3://bucket/somefolder/` with a local directory structured as
- dir
- myfile

The result now is an s3 path of s3://bucket/somefolder/dir\myfile. The expected path is s3://bucket/somefolder/dir/myfile.

Reproduce the issue with the documented `s4cmd -r put dir s3://bucket/somefolder/` command on Windows, using the shown directory layout. Trace the recursive upload path construction; done means the uploaded key uses forward slashes, producing `s3://bucket/somefolder/dir/myfile` rather than a backslash-separated path.

Written by the indexing model from the issue text.