fixes bug 1067548 - API testdrive binary data and failure after success

@rhelmer r?

Now it looks like this instead:
![screenshot 2014-09-15 14 49 39](https://cloud.githubusercontent.com/assets/26739/4279520/3e85e70a-3d22-11e4-9d36-99e5800473e8.png)

I was cheeky and extended the functionality slightly by adding the "Size: XXX.X Kb" output too. I think this helps to be able to tell what was returned. 
Well, it doesn't go too badly. See the [screenshot](https://bug1067548.bugzilla.mozilla.org/attachment.cgi?id=8489585)
Also, this is what happens when you do `less foo.png` and view it in the terminal. 
Another more interesting option would maybe be to allow the user to download it at that point. Basically, redirecting the user to the URL of the endpoint you just tested. 

How about we keep the "See it anyway?" and add "Download the content?" next to it?