Update Controller/ImagineController.php
Update Controller/ImagineController.php

Set format to null if not defined by the options. 
samrodriguez/AvalancheImagineBundle,hephaistosthemaker/AvalancheImagineBundle,ollietb/AvalancheImagineBundle,avalanche123/AvalancheImagineBundle,0x2f8f/AvalancheImagineBundle,jmcclell/AvalancheImagineBundle,analogic/AvalancheImagineBundle,snpy/AvalancheImagineBundle