ETIMEDOUT causes uncaught exception error

I hit to timeout error which causes uncaught exception error (error: uncaughtException: Cannot read property 'headers').  The reason was that the error handler try to use response.header variable but response was null.

The problem is in [lib/datasift.js on line 152](https://github.com/datasift/datasift-node/blob/master/lib/datasift.js#L152).

I made this quick fix to avoid uncaught exception error.