Changed extra_*_kwargs default value to `None`, to protect against bugs.
Changed extra_*_kwargs default value to `None`, to protect
against bugs.

jupyter/dockerspawner,minrk/dockerspawner,ibab/dockerspawner,Xarthisius/dockerspawner,Xarthisius/dockerspawner,jhamrick/dockerspawner,jlamcanopy/dockerspawner,bigzachattack/dockerspawner,Fokko/dockerspawner,betatim/dockerspawner,jupyter/dockerspawner,anaderi/dockerspawner,ibab/dockerspawner,bigzachattack/dockerspawner,quantopian/dockerspawner,betatim/dockerspawner,quantopian/dockerspawner,jhamrick/dockerspawner,minrk/dockerspawner,jlamcanopy/dockerspawner,anaderi/dockerspawner,Fokko/dockerspawner