Filename of downloaded assertions zip isn't set properly

[vteague]

When the assertions zip is downloaded, in either json or csv format, its filename isn't properly set.

Expected behavior:
The downloaded file should be called assertions.zip

Observed behavior:
On firefox, the downloaded file is named after the endpoint: get-assertions.
On chromium, the downloaded file is named get-assertions.zip

[vteague]

Fixed as part of #137.

[vteague]

Fixed in #137.