Use filename if not provided when doing dataset copying

A file put with `datasets.put` of a single file needs explicit filename in the path, but should be able to use the existing file name for smarter uploads.

Eg. a call like
```
dataset.put("/project/data.csv", "/somefolder/")
```
results in `/somefolder/` being created (if it doesn't exists), but no file uploaded there. I think if done smartly that above should be equivalent to explicitly writing
```
dataset.put("/project/data.csv", "/somefolder/data.csv")
```
which is just getting the current file name and adding to the end of the path, if not provided.

If that auto filename is not desirable for some reason then the upload should at least error out, otherwise confusing that things succeed and nothing is copied.