`DownloadHandler` should allow returning `Path` objects

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

Start at the DownloadHandler implementation and trace how the value returned by a download function is handled when it is a string. Add support for pathlib.Path values and enforce absolute paths, then add or update coverage for both behaviors and run the relevant test suite.