DownloadHandler should allow returning Path objects

[wch]

Currently, DownloadHandlers can accepts strings, but not Path objects, so they have to be used like this:

@render.download()
def download():
    path = Path(__file__).parent / "mtcars.csv"
    return str(path)

It would be more convenient if you could just return a Path object, like this:

@render.download()
def download():
    return Path(__file__).parent / "mtcars.csv"

Also, I believe that currently there's no checking that the returned path is absolute, but we should do that, because the current working dir may not be the same as the app directory