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 comment is confusing me, sorry :) If it was binary before this run, then hide ```.binary-response-warning```?