Ensure mirror input field is correctly updated when the source input field is empty...

Hello,

I am integrating this component into a component I wrote (https://github.com/jblashill/recipients.ui.js) and came across this issue. Luckily the fix was an easy one:

AutosizeInput.update() doesn't correctly update the mirror when the source input field is empty.

(Tested on Chrome 27)

Thanks,
James