Fix center minzoom maxzoom action

Addresses https://github.com/mapbox/mapbox-studio/pull/857#issuecomment-56056783.

Note that in no cases will this autoadjust the center to fit within min/maxzoom bounds if `saveCenter: false`. You'll get an error and have to realize that your center needs to be unlocked.

cc @ian29 @samanpwbb