Copy to non-existing S3 folder path creates a file with "/" instead of file name in that folder

Steps to reproduce:
```python
# take an existing file
src = S3Path("s3://my-bucket/file.txt")
# and copy it into non-existent folder
dst = S3Path("s3://my-bucket/destination/")

src.copy(dst)
```

**Current behavior**

Creates a file with name `/` inside the folder `s3://my-bucket/destination/`

**Expected behavior**

The file `s3://my-bucket/destination/file.txt` is created