changed a return to return cb(null)

Hey,

this commit seems to fix the bug described here. https://github.com/balderdashy/waterline/issues/282

I only checked the update method. If the save method does not rely on the update method there may be another bug there, but they look very similar to me.

I got this (small) fix working in my code, but u still might want to ram it through the test machinery just to be sure it does not break anything i dont know about.

Hope this helps. 
Kind Regards