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. 
This seems like a footgun... I can't think of any conceivable reason to display the binary output directly in the browser. Best case it's not useful for anything, worst case it locks up the browser.