Accept URI in download function

[mfsch]

The request-based functions (e.g. HTTP.get) accept the URI type for the url argument, but the download function currently only accepts strings. This (tiny) change allows URIs there too.

[fredrikekre]

Looks good, can you add a test?

[mfsch]

I’ve added a test – however, it looks like the tests for download were commented out in #1065. Maybe @quinnj remembers why? Commenting them in an running Pkg.test("HTTP", test_args = ["download.jl"]) runs without error for me.