File download fails when client has an AV gateway in place

[epimeth]

Hi John!

First of all, thank you for the great library. I've had no problems with it at all.
Except for this, of course :-)

It has only come up a few times, and I'm not quite sure that it is worth the time to fix, but if my higher-ups decide to make this a priority in any way I'll do so and send you a PR if you don't do it yourself.

So here's the situation. Some of our clients have a gateway in place that intercepts file downloads and potentially displays an HTML page instead. Specifically I can recall McAfee Web Gateway. This is an AV system that intercepts the download and scans it for viruses, displaying a page with a progress indicator and a "download" button when complete.

This functionality is entirely incompatible with v1.4.3 - I have not tried to update to verify if you already have a fix in place, so please let me know if there is. Also, there is no changelog since 1.4.1 and as far as I'm concerned "if it aint broke don't fix it" so I haven't updated ¯_(ツ)_/¯

Let me know what you think!