Stop mangling filenames when downloading them

Summary: Fixes T6990. We unnecessarily mangle filenames when downloading them.

Test Plan:

• Reviewed "quoted-string" grammar in HTTP spec (http://www.w3.org/Protocols/rfc2616/rfc2616-sec2.html)
• Downloaded some non-latin binaries.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6990

Differential Revision: https://secure.phabricator.com/D11940