Don't url escape filenames on download

[gnestor]

Closes #2742

[takluyver]

This looks fine to me, but I'd like @minrk to have a quick look too. I notice you've also marked it for 5.2, so we should release 5.1 before merging it.

[gnestor]

We can fit this in 5.1 as long as we're confident that this won't break anything else! 😮

[minrk]

This solves a problem for spaces in ascii names but reintroduces a problem for non-ascii names.

Test with a filename like 'üni spåce™' to be sure it's working.

[minrk]

Some diagnoses:

1. the + was introduced by using the wrong escape function (tornado.escape.url_escape, which is for URL query parameters by default, not url components)
2. Putting a name in quotes means it shouldn't be escaped, but it also has to be Latin-1 if you do that.
3. RFC 6266 explains how to specify filename encodings properly (url escapes were the right thing to do, we just did them wrong)

#2767 should implement filename encoding correctly. I've tested it on Safari, Chrome, and Firefox.

[takluyver]

Closing in favour of #2767.