make the curl plugin respect Content-Disposition

Summary:
The `Content-Disposition` header is what tells the browser what name
to give the file it's downloading. We weren't respecting that before.

Test Plan:
Create `test_curl_plugin.py`. We should probably separate other plugin
testing into more specific files too.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D38