This commit allows url with adjacent slashes to be handled correctly.

- Disabled RestProtocolURICleaning (link) as it do not allow adjacent slashes.
- Changed path.Join method to strings.Join when joined remote urls. path.Join also cleans paths which removes additional slashes.
- Kept local urls to be joined with path.Join.
- Added test cases for copy;
    - Remote to local
    - Local to remote
    - Remote to remote
- Removed redundant trailing slashes in the keys of s3 objects in ls_test.go and sync_test.go files.

Fixes: peak#352 ,peak#449

Replaced strings.Cut method as it is undefined before go version 1.18.x