Allow drop_upload to put file directly in folder

[ClaytonJY]

In order to upload a file into a folder without giving it a different (base)name, a user has to build the path themselves, e.g. drop_upload(file, paste0(folder, "/", file)). This happens a lot in our tests, too.

I think we should be able to do this in the function, as in drop_download.

The trick is that we'd have to check if the path argument is a dropbox folder. The right way to do that is probably to use drop_get_metadata, and if it returns metadata, check the .tag. Then we could make functions like drop_is_folder and drop_is_file, which would look a lot like drop_exists but with an additional check for the tag.

[karthik]

This is totally doable. 😴 now. But before I head to sleep:

These functions (https://github.com/karthik/rdrop2/blob/master/R/drop_file_ops.R#L293-L317) will give you what you need to get a TRUE or FALSE on drop_is_folder. If TRUE, then we append the filename to the path and push it there.

You can see this working in drop_copy and drop_move.