[APPS-392] No longer escaping XML content before sending over to Freshbooks

Looks like we now already escape content going through the proxy - so this app was causing the XML to double escape. Removing the escaping (which is what this PR does) immediately alleviates the problem described below

> Freshbooks app returns error when looking up users: 
> ![](http://f.cl.ly/items/0i3x2E0G451r1q0C3i2Z/Screen%20Shot%202013-03-14%20at%2011.43.03%20AM.png)
>
> That error was caused by the Freshbooks API returning this on all our calls:
>
> ```
> <?xml version="1.0" encoding="utf-8"?>
> <response xmlns="http://www.freshbooks.com/api/" status="fail">
>   <error>Your XML is not formatted correctly.</error>
>   <code>40010</code>
> </response>
> ```